Boat Winterization Checklist by System (2026) | CritPro

A practical boat winterization reference for outboards, inboards, sterndrives, marine plumbing, sanitation, batteries and long-term storage. Includes a printable system checklist and spring restart log.

Winterizing a boat is less about following one universal procedure and more about identifying every component that can be damaged by freezing, corrosion or months of inactivity. Water trapped in an engine cooling passage, washdown pump or hose fitting can expand during a freeze. Meanwhile, untreated fuel can deteriorate, batteries can lose charge, and moisture trapped below deck can lead to mildew or corrosion.

The challenge is that two boats of the same length can require very different work. A 19-foot outboard-powered skiff may have only an engine, livewell, bilge pump and battery to consider. A cabin cruiser can add an inboard or sterndrive, generator, water heater, pressurized freshwater plumbing, marine toilet, holding tank, air conditioning and shore-power system. A single forgotten system can undo an otherwise careful layup.

This CritPro reference organizes preparation by system rather than by a generic sequence of chores. Use the overview to identify what is installed on your boat, read the relevant section, and mark each item on the printable checklist before storage. For engines, watermakers, refrigeration, heating and other equipment, the manufacturer's instructions take priority over general guidance here.

Boat Winterization at a Glance

Start by making an equipment inventory. A system only needs a check mark after its own winterization requirements have been completed and verified. The table identifies typical concerns, not a universal installation procedure.

A boat may have multiple separate raw-water, freshwater and drainage circuits. Do not assume protecting the main engine or freshwater pump protects every other onboard system.

The Order of Work: A Winter Layup Workflow

A logical order helps prevent rework. For example, fuel treatment and engine flushing generally happen before the engine is shut down for the season, while plumbing protection should be completed before temperatures fall below freezing.

Gather the relevant manuals, a notebook or worksheet, replacement seals and filters, the correct lubricants, a pump-out appointment if needed, and an approved means of collecting waste fluids. Work on the boat in a well-ventilated area, prevent spills, and use a marine technician for engine or through-hull procedures you cannot complete safely.

Winterizing Outboard Motors

Many outboard motors are designed to drain cooling water when they are placed in the correct storage position. That does not mean an outboard needs no winterization. Fuel, gear-case lubricant, internal corrosion protection and the correct tilt position remain important, and the exact service procedure differs by model, age and fuel-injection system.

Fuel and Engine Service

Check the manufacturer’s storage procedure before treating fuel or running the engine. Mercury recommends stabilizing fresh fuel and circulating treated fuel through the system, with special consideration for older vented tanks compared with newer low-permeation tanks. Change four-stroke engine oil and filters at the prescribed interval; leaving contaminated oil in an engine over a long layup can expose internal surfaces to moisture and combustion byproducts.

Fogging and internal corrosion protection are not identical across engine designs. Conventional two-strokes, direct-injection engines and four-strokes may require different products or procedures. Do not spray oil into a modern fuel-injection intake or disable engine sensors unless the engine maker explicitly directs it. If you are uncertain, have an authorized service facility handle the internal engine treatment.

Gearcase, Propeller and Storage Position

Change or inspect lower-unit gear lubricant as specified. Milky or creamy gear oil can indicate water intrusion, and storing a contaminated gearcase through freezing conditions may cause damage. Inspect the propeller shaft, fishing-line buildup, anodes and serviceable grease points while the boat is out of the water.

After flushing and completing the appropriate service, store the motor in the manufacturer-recommended position. Many conventional outboards should be left fully lowered or vertical so cooling passages can drain, but confirm the requirements for your specific outboard and installation. Never run an engine without the cooling-water supply required by its owner’s manual.

Mercury and Yamaha publish model-oriented seasonal maintenance guidance. See Mercury’s outboard winterization guide and Yamaha’s boat winterizing recommendations.

Inboard Engines and Sterndrives: Protect Every Cooling Circuit

Inboards and sterndrives require special attention because water can remain in low points, strainers, engine blocks, manifolds and exhaust components. Some installations are raw-water-cooled while others use a closed-loop coolant circuit plus a separate raw-water circuit for the heat exchanger and exhaust. Protecting the closed coolant does not automatically protect the raw-water side.

Raw-Water-Cooled and Closed-Cooled Engines

Follow the engine maker’s approved draining or antifreeze procedure, accounting for all drains, hoses, coolers, heat exchangers, manifolds, risers and strainers in the installation. If the method uses antifreeze circulation, the required product, concentration, supply method and engine operating steps are installation-specific. Seeing colored antifreeze at one outlet does not, by itself, prove that every branch of the cooling system is protected.

For closed cooling systems, verify coolant condition and freeze protection using the manufacturer's specified mixture and test method. Never substitute potable-water-system antifreeze for the correct engine coolant, and never assume the two sides of a heat-exchanger installation use the same product.

Sterndrive-Specific Checks

Inspect the outdrive lubricant for water contamination, examine bellows and accessible seals, check anodes, and complete gearcase maintenance before storage. Depending on the model, the engine, power-steering cooler, gearcase and drive may have separate maintenance or draining requirements. Water found in a drive lubricant reservoir warrants diagnosis before the boat is put away.

An oil extraction setup can help on engines where access to the oil pan is limited. CritPro carries the Jabsco DIY Engine Oil Change System, which uses a pump and collection bucket for controlled oil changes. Always collect oil and coolant in suitable containers and take them to approved recycling or disposal facilities.

Fuel Tank and Fuel System Preparation

Winter storage can allow gasoline to oxidize, form deposits or absorb water, while diesel systems face their own moisture and microbial-contamination concerns. Fuel treatment, tank level and filter service should follow the engine and fuel-system manufacturers' recommendations, especially for ethanol blends and tank designs.

Older vented gasoline tanks and newer low-permeation tanks should not automatically be treated the same way. For example, Mercury recommends leaving expansion space in an older vented tank when filling for storage, while noting that newer tanks do not necessarily need the same fill strategy. Do not top off a tank to the neck, improvise chemical treatments, or assume a stabilizer restores spoiled fuel.

If the engine maker calls for fuel treatment, add the approved stabilizer at the specified dose and circulate it through the system as instructed. Inspect water-separating filters and record any signs of contamination. If fuel smells degraded or has visible water or debris, seek service rather than relying on an additive to solve the underlying problem.

Freshwater Plumbing and Water Heater Winterization

Pressurized freshwater systems can trap water in a surprising number of places: strainers, pumps, accumulator tanks, faucets, mixer valves, cockpit showers, transom showers, water heaters and the short hoses leading to them. Even if the main tank is drained, low points or check valves can hold enough water to crack a fitting during a freeze.

Why the Water Heater Is a Separate Step

Isolate power to an electric water heater and allow it to cool before draining. Drain the heater by the manufacturer's procedure and use a compatible bypass arrangement when the manufacturer permits it. A bypass protects the system’s hot and cold supply lines without filling an entire heater tank with winterizing fluid. Never energize a drained electric water heater, because its heating element can be damaged.

Conceptual illustration only. Valve placement and bypass operation vary by boat. Follow the water-heater and plumbing manufacturers’ procedures before changing any connections.

Protect Every Fixture and Low Point

After draining as completely as practical, protect remaining freshwater circuits using a product expressly intended for potable marine water systems, where the equipment manufacturer permits that approach. Work through the cold and hot sides of each faucet, shower and deck wash point. Include the pump strainer and accumulator if present. Account for appliances such as ice makers, water filters and reverse-osmosis equipment individually; many require special bypass or storage procedures and should not simply be filled with generic antifreeze.

Do not use ethylene-glycol automotive engine coolant in drinking-water plumbing. A non-toxic propylene-glycol marine/RV winterizing product may be appropriate for many systems, but its marked freeze or burst-protection rating, compatibility and dilution requirements matter. The color of the product is not a substitute for checking the label and the manufacturer's instructions. See West Marine’s potable-water winterization guidance for additional system considerations.

Marine Head, Holding Tank and Sanitation Lines

Marine toilets and sanitation plumbing may have their own intake, flushing, discharge and holding-tank circuits. Pumping the holding tank empty is an important first step, but it does not necessarily remove water from the toilet pump, intake line, treatment equipment or macerator.

Schedule a legal pump-out and flush the system as directed. Then winterize the toilet’s water-containing parts, hoses and any macerator or treatment device according to that equipment’s manual. Use only products compatible with seals, gaskets, hoses and any treatment chemistry; do not mix household cleaners or chemicals in the sanitation system. If the boat remains afloat, verify every seacock’s required operating and storage position and keep a record of it.

A pressurized freshwater toilet may need attention to both its freshwater supply and sanitation discharge. A raw-water-flush toilet may instead have a separate intake pump or manual pump body that needs protection. Label any closed valves and document what must be reopened before spring use.

Livewell, Washdown, Air Conditioning and Other Hidden Water Circuits

Some of the most easily missed freeze points are accessories rather than engines. Raw-water washdown pumps, livewell circulation systems, baitwell pumps, fish-cleaning stations, cockpit showers, deck drains, shower sumps, raw-water-cooled marine air conditioning and refrigeration equipment may each retain water in a dedicated loop.

Trace each intake to its pump, valves, strainers, hoses and discharge or overboard outlet. Drain or protect the complete circuit according to the equipment maker; running winterizing fluid through one fixture does not necessarily fill a separate branch or a dead-end hose. For reverse-cycle air-conditioning, generators and watermakers, consult the manufacturer or a qualified technician because internal components and approved storage solutions vary substantially.

Where a system has a sea strainer or through-hull, follow the boatyard and manufacturer's procedures for winter storage. A boat left afloat has different seacock and bilge requirements from one stored on a trailer. Never alter a through-hull configuration casually or leave a required water-exclusion valve open solely for convenience.

Batteries, Shore Power and Bilge Pump Readiness

A battery that sits partly discharged through a long storage period may be difficult to recover, and battery care varies by chemistry. Flooded lead-acid, AGM and lithium batteries do not all have the same charging and storage requirements. Confirm the owner's manual for the battery, charger and any battery management system before deciding whether to disconnect, maintain, or remove the battery.

Boats Stored on Land

Fully charge lead-acid batteries according to their specifications and inspect terminals for corrosion. If the boat will have no electrical loads, a suitable disconnect or battery removal may be appropriate. When using a charger or maintainer, match its charging profile to battery chemistry and confirm that the storage location and power supply are suitable for unattended maintenance. Lithium batteries require particular attention to manufacturer-prescribed storage state of charge and low-temperature charging limits.

CritPro carries the Guest 10A 12V 2-Bank On-Board Battery Charger for compatible battery systems. Our Marine Battery Charger Buying Guide explains bank count, charging profiles and battery-chemistry compatibility in greater detail.

Boats Remaining Afloat

Do not simply disconnect every battery on a boat that remains in the water. Automatic bilge pumps, monitoring devices, alarms or other essential equipment may need continuous power. Establish a battery and shore-power maintenance plan suitable for the installation, and arrange regular inspections by a responsible person or marina. A bilge pump cannot replace safe mooring and inspection practices.

Clean the bilge, inspect discharge hoses and verify manual and automatic pump operation before storage. If a pump does not start, runs without moving water or cycles unexpectedly, use our Bilge Pump Troubleshooting Guide to help identify the cause before the boat is left unattended.

Hull, Deck, Trailer and Storage Cover

Clean and dry the boat before covering it. Remove food, bait residue and damp gear, and check lockers, livewells and coolers for remaining water. Clean upholstery and dry removable cushions and life jackets before storing them in a dry, ventilated place. For compatible fiberglass and gelcoat finishes, complete needed washing and protection before the boat is put away; see our Boat Cleaning and Detailing Guide.

Inspect the cover or shrink-wrap arrangement for ventilation, drainage and support. A cover that collapses into the cockpit can hold rain or snow and overload fabric, hardware or supports. A Camco Adjustable Telescoping Boat Cover Support Kit is one type of accessory used to help support suitable covers, but load capacity, fit and weather exposure still matter.

For trailered boats, examine tires, hubs, bearings, brakes and lighting, and follow trailer manufacturer guidance for long-term storage and wheel loads. Make sure the trailer is positioned securely on suitable ground, the boat is properly supported and drain plugs are handled as recommended for the storage setup. Keep the plug or required reinstall reminder in a clearly visible location.

For damp cabins or enclosed storage compartments, a moisture-control product such as the Star Brite No Damp Moisture Absorber may be useful where the manufacturer approves its use. Moisture absorbers do not replace drying the boat, fixing water leaks or maintaining safe ventilation.

Winterization Priorities by Boat Type

Not every boat has every system. Use this comparison as a reminder to add the equipment that is actually installed rather than following the shortest list that appears to fit the boat.

Printable Boat Winterization Checklist

Print this section or copy it to a maintenance log. Mark N/A for equipment your boat does not have, but never mark a step complete until the relevant procedure has been performed and checked. Record product types and quantities in your own service log rather than assuming another owner's values will fit your boat.

Frequently Asked Questions About Winterizing a Boat

Do I need to winterize a boat in a warm climate?

Not every warm-climate boat needs the same freeze protection as a boat stored through a northern winter. However, occasional freezes, extended inactivity, saltwater corrosion, stale fuel and battery discharge can still cause problems. Use the actual storage conditions, manufacturer instructions and expected exposure to determine what must be protected.

Can I just drain my freshwater tank?

Usually not. Pumps, valves, strainers, showers, hoses and water heaters can retain water after the tank has emptied. Protect all components using the approved drainage or antifreeze method for your system.

Is RV antifreeze safe for every boat system?

No. Potable-water-system winterizing products and engine coolant serve different purposes, and marine pumps, heads, seals and watermakers may have specific compatibility restrictions. Check the actual product label and equipment manuals before use. Never use toxic automotive ethylene-glycol coolant in drinking-water plumbing.

Should I fill my fuel tank completely for winter?

Not as a universal rule. The recommended fuel level depends on tank design, fuel type and manufacturer guidance. Some older vented tanks benefit from being kept mostly full with expansion room; other designs have different needs. Follow the engine and tank manufacturer's procedure.

Should I disconnect my boat batteries?

For a boat stored ashore with no required onboard loads, disconnecting or removing the battery may make sense after proper charging and preparation. For a boat left afloat, bilge pumps, alarms and monitoring equipment may need uninterrupted power. In either case, follow battery-chemistry and charger instructions.

How do I know whether I missed a water system?

Inventory every pump, tank, drain, inlet, fixture and outlet. Compare that inventory with the boat's actual plumbing rather than relying only on the number of sinks or engines. Separate circuits for livewells, washdown, air conditioning, freshwater and sanitation are common sources of omissions.

Technical References and Further Reading

These outside resources explain important parts of seasonal layup and help owners locate the correct manufacturer procedures. They should be read together with the model-specific manuals for the equipment aboard.

• Mercury Marine: How to Winterize Your Outboard, outboard maintenance and storage
• Yamaha Outboards: Boat Winterizing Tips, seasonal preparation and battery storage
• West Marine: Winterizing Potable Water Systems, water heaters and plumbing
• BoatUS: Winterization Resource Library, engine and storage risk considerations
• Sailors for the Sea: Winterizing Your Boat, lower-impact maintenance and waste disposal

Final Thoughts: Protect the Systems, Not Just the Engine

Effective boat winterization comes down to identifying all the places water, fuel, electricity and moisture interact with the boat during storage. The engine is important, but it is only one part of the job. Pressurized freshwater plumbing, marine heads, livewells, washdown pumps, air conditioning, bilges and batteries can all require separate attention.

Use the system matrix to decide what is installed, follow equipment-specific procedures, and keep a completed checklist for spring. The extra documentation can make recommissioning easier because you will know which valves were closed, which systems were treated, and which problems still need attention before launching.

For related maintenance and safety guidance, see CritPro's Boat Safety Gear Checklist and browse our on-board marine battery chargers when maintaining a compatible battery system during the off-season.