A boat winterization checklist covers four jobs: treat the fuel, protect the engine, remove water from anything that can freeze, and store the boat covered with the battery looked after. Mercury Marine says to winterize once temperatures consistently dip below 50°F and to finish before the first hard freeze.
At a glance
- Start when temperatures consistently dip below 50°F, and finish before the first hard freeze, as Mercury Marine advises.
- Outboards, sterndrives and personal watercraft are winterized differently, so the engine type decides which cooling steps you follow.
- Sterndrive cooling systems and onboard water lines get propylene glycol antifreeze, while a Sea-Doo's closed-loop cooling system already contains antifreeze.
- Fuel gets a marine stabilizer and a full or nearly full tank: Mercury says to leave room for expansion, and Sea-Doo says to fill a watercraft's tank completely.
- Batteries come off the boat or go on a maintainer, and the boat goes under a breathable cover or recyclable shrink-wrap.
If you are shopping for a boat over the winter, ask each seller how and when it was winterized. Browse boats for sale from dealers on Searchshop.
The engine type changes the cooling and fogging steps, while fuel, battery and cover steps apply to almost every boat.
| Task | Outboard | Sterndrive | Sea-Doo PWC |
|---|
| Fuel stabilizer and fresh fuel | Yes | Yes | Yes |
| Cooling water | Flush, then trim fully down to drain | Drain seawater system, fill with propylene glycol | Blow out exhaust water; closed-loop coolant stays |
| Fog the engine | Yes, per manual | Per engine manual | Yes |
| Oil and filter | Every 100 hours or once a season (four-stroke) | Before long-term storage | Change oil and filter |
| Gear lube | Change; check for water | Change; check for water | Not listed by Sea-Doo |
| Storage position | Trimmed down | Drive tilted down if stored outdoors | Dry, ventilated place |
Sources: Mercury Marine outboard and MerCruiser pages, Sea-Doo's winterizing guide. The MerCruiser page does not cover fogging, so follow your sterndrive's manual for that step.
Every figure and recommendation attributed below comes from an engine or boat maker's own instructions or from a state or Sea Grant clean-marina guide, read on 26 October 2026; the planning tips in the Next steps section are Searchshop's own suggestions. The sources are Mercury Marine (outboards and MerCruiser sterndrives), Yamaha Outboards, Sea-Doo, the Massachusetts Clean Marina Guide and New York Sea Grant. Where two sources give different advice, both are shown. Your engine's owner's manual comes first: Mercury points owners to the operator's manual for model-specific steps such as fogging.
Winterize once the temperature consistently dips below 50°F, and finish before the first hard freeze. That is Mercury Marine's guidance, and the company lists waiting too long as the first of seven winterizing mistakes because freezing water and condensation can crack lines and damage the engine.
The fuel system needs a marine fuel stabilizer, a nearly full tank and a short run to circulate the treated fuel.
- Add stabilizer. Mercury says to use a stabilizer approved for marine engines and to run the engine for 10 to 15 minutes so treated fuel reaches the whole system. New York Sea Grant adds a caution: check the engine's warranty before adding fuel stabilizers.
- Fill the tank, but not to the top. Mercury's outboard advice is to fill to about 95 percent before long-term storage, leaving room for expansion. The Massachusetts Clean Marina Guide recommends 80 to 90 percent and says not to go above 90 percent, so fuel does not spill out of the vent in spring.
- Replace the fuel filter. Mercury's outboard guide makes a new fuel filter the last fuel step, and its MerCruiser guide says to replace the water-separating fuel filter.
- Check for special procedures. MerCruiser says its MPI sterndrive engines need a special mix of fuel, stabilizer and lubricant to fill the fuel delivery system, so follow that engine's manual.
An outboard needs flushing, fogging, fresh lubricants and a fully trimmed-down position so the cooling system drains.
- Flush and drain. After the last run, flush the engine with fresh water, then tilt or trim it all the way down so trapped water drains. Mercury says the engine should be completely dry before winter storage.
- Fog the engine. Mercury says to treat four-stroke and conventional two-stroke engines with fogging oil to prevent corrosion. Its DFI two-stroke engines get one ounce of DFI outboard oil per cylinder through the spark plug hole instead. Follow the fogging procedure in your operator's manual.
- Change oil and filter. Mercury lists four-stroke oil and filter changes at every 100 hours or once a season, and warns that used oil contains acids and contaminants that can corrode internal engine parts over time.
- Change the gear lube. Water in the lower unit can freeze and crack the gearcase. Mercury says white or creamy gear lube may mean water is present. Yamaha fills its gearcases from the bottom drain hole until lube weeps from the upper vent, and warns not to fill from the top.
- Grease and protect. Lubricate every grease point in the owner's manual, check behind the propeller for fishing line, and grease the prop shaft before reinstalling the propeller.
- Check anodes and trim fluid. Mercury says to replace sacrificial anodes when they are 50 percent deteriorated, and that power trim fluid should be bright red; pink or milky fluid points to water and a dealer inspection.
Searchshop's Yamaha vs Mercury outboards comparison covers how the two makers' warranties and maintenance advice differ.
A sterndrive needs its seawater cooling system drained and refilled with propylene glycol, plus fresh oil and gear lube. Mercury's MerCruiser guide sets out these steps:
- Change the engine oil and filter before long-term storage, even if the season's use is not near 100 hours.
- Drain the engine's seawater cooling system following the instructions for your engine, then fill it with propylene glycol mixed to the manufacturer's recommendation. MerCruiser says it is very important to displace all water from the cooling system.
- Change the sterndrive gear lube and look for water in the old lube, because water can freeze and damage the gearcase.
- Lubricate the engine coupler and the propeller shaft splines, and check the prop shaft for fishing line.
- Store the boat with the drive tilted down if it stays outdoors, so rain and snow do not collect in the drive.
Inboard engines are not covered by the MerCruiser sterndrive guide, so use your inboard engine maker's storage procedure. Searchshop's guide to inboard vs outboard vs sterndrive power explains how the three layouts differ.
A Sea-Doo personal watercraft needs its exhaust system drained, its engine fogged and its battery kept charged, but its cooling system does not need antifreeze. Sea-Doo says its closed-loop cooling system already contains antifreeze and does not require flushing.
- Fill the tank completely with fresh gasoline to limit condensation and add fuel stabilizer according to the label.
- Change the engine oil and filter, using the operator's guide for details.
- Drain the exhaust system by running the engine at idle on a hose, then blowing out the water with compressed air. Sea-Doo warns not to exceed 55 PSI.
- Spray storage (fogging) oil into each cylinder.
- Disconnect the battery, negative terminal first, and keep it on a maintainer. Sea-Doo says a drained battery can freeze and fail.
- Wash the hull and deck with a marine-safe cleaner, then store the craft under a breathable, fitted cover in a dry, ventilated place. Sea-Doo advises against wrapping it in plastic, which can cause mold.
Other personal watercraft brands have their own procedures, so check the owner's manual before copying these steps.
Onboard plumbing should be drained as completely as possible, with propylene glycol antifreeze only where water cannot be removed. The Massachusetts Clean Marina Guide says to drain as much water from the water system as possible rather than relying on antifreeze, and to use a diluted, water-system-approved antifreeze such as propylene glycol in traps that cannot be drained. New York Sea Grant says propylene glycol, usually pink, is less toxic than ethylene glycol, and its engine decommissioning page says to use the minimum amount needed. It also recommends inspecting and cleaning bilges before extended storage.
The battery should come out or stay on a maintainer, and the boat should go under a cover that keeps out snow, debris and moisture.
- Battery. Mercury says to check the fluid level in lead-acid batteries and top up with distilled water if needed, charge the battery fully, remove it and store it in a cool, dry place on a maintenance charger. Mercury also says never to leave a battery connected in a stored boat, especially outdoors.
- Cover. Mercury recommends a high-quality breathable cover or shrink-wrap to keep out debris and prevent mold and mildew, secured tightly and checked during the season. The Massachusetts guide encourages canvas covers or recyclable shrink-wrap.
- Corrosion protection. Mercury suggests a rust inhibitor on the powerhead, especially for saltwater boats, and touching up paint nicks on the lower unit.
Where the boat spends the winter matters too. Searchshop's boat storage guide compares wet slips, dry stack and trailer storage.
Used oil, antifreeze, batteries and stale fuel should go to a recycler or a hazardous waste program, not the trash or the water. The Massachusetts Clean Marina Guide classifies oil, solvents, antifreeze and batteries from engine maintenance as hazardous waste. New York Sea Grant says stale gasoline that cannot be reconditioned should be disposed of as hazardous waste, that waste gasoline is not considered hazardous if it is recycled or burned as fuel, and that household hazardous waste programs may accept unwanted gasoline from boat owners.
Use the checklist in three passes: decide which engine section applies to your boat, gather the supplies your owner's manual names, and finish before the first hard freeze.
- Doing it yourself: follow the fuel section, your engine's section and the battery and cover section in that order, and write down the date and what you did for next spring.
- Using a dealer or marina: Sea-Doo points owners to its dealers for professional winterization. If someone else will do the work, book it early, since the deadline is the first hard freeze.
- Buying a used boat: ask for the winterization record and check it against this list. Searchshop's boat maintenance guide covers the rest of the yearly service.
Thinking about a different boat for next season? Compare boats for sale on Searchshop and set a price-drop alert on the ones you like.
Winterize your boat once temperatures consistently dip below 50°F, and finish before the first hard freeze. That is Mercury Marine's advice. Waiting until snow is on the ground raises the risk that water left in the cooling system, gearcase or plumbing freezes and cracks something.
Outboard cooling systems are drained rather than filled with antifreeze in Mercury's outboard winterizing steps. After a fresh-water flush, Mercury says to trim the motor down to its vertical position so remaining water drains out, and to make sure the engine is dry before storage. Onboard plumbing on the same boat may still need propylene glycol where water cannot be drained.
Yes, fill the tank most of the way with stabilized fuel, but leave room for expansion. Mercury recommends about 95 percent full for long-term storage. The Massachusetts Clean Marina Guide recommends 80 to 90 percent and no more than 90 percent, so fuel does not spill from the vent when it warms in spring. Add a marine fuel stabilizer and run the engine so the treated fuel circulates.
Propylene glycol antifreeze is used for boat winterizing. Mercury's MerCruiser guide says to fill a sterndrive's drained seawater cooling system with propylene glycol mixed to the manufacturer's recommendation. New York Sea Grant says propylene glycol, usually pink, is less toxic than ethylene glycol and should be used in the minimum amount needed. New York Sea Grant makes an exception for closed, or freshwater, cooling systems.
