Winterizing a boat means two things: protecting the engine and fuel system during months of sitting, and getting water out of anything that can freeze and crack. The steps below come from engine makers (Mercury and Yamaha), Sea-Doo, and the BoatUS winterizing guide. Your engine's own operation and maintenance manual has the exact procedure for your model, and many owners have a dealer do the job. All sources were checked on October 27, 2026.
Fuel
- Start with fresh, stabilized fuel. Mercury says most of the problems its dealers see come from stale fuel. Run the tank nearly empty at season's end, then add fresh fuel (ethanol-free if available) treated with a marine fuel stabilizer. If the fuel is more than a month old and you can't use it up, Mercury suggests having it pumped out and replaced. Mercury: How to Winterize Your Outboard
- Fill vented tanks to about 95%. On older boats with vented tanks, Mercury says to fill with fresh stabilized fuel to stop condensation, but stop at about 95% so the fuel has room to expand. Newer tanks that can't vent freely to the air don't need to be filled. Mercury: How to Winterize Your Outboard
- Circulate it. Run the engine about 10 minutes, in the water or on a garden-hose flush device, so treated fuel reaches the whole system, then replace the fuel filter. Mercury: How to Winterize Your Outboard
- Don't cap the vent. Mercury's FAQ says not to cap the tank vent or fill to overflowing. Mercury Service & Support FAQs
- Portable tanks: BoatUS suggests emptying fuel from portable four-stroke tanks into your car's tank and taking the tanks home. BoatUS: The Boater's Guide to Winterizing (PDF)
Outboard engines
From Mercury's outboard winterizing guide and the BoatUS checklist:
- Fog the engine. While it's still warm, remove the spark plugs on four-stroke and conventional two-stroke engines and spray fogging oil into each cylinder. On direct-injected (DFI) two-strokes such as Mercury OptiMax, Mercury says to squirt one ounce of DFI outboard oil into each cylinder instead of fogging oil. Mercury: How to Winterize Your Outboard
- Flush, drain the gearcase and add fresh lubricant, and inspect the anodes. BoatUS also says to store the outboard in its lowest (tilted down) position. BoatUS notes some newer outboards have an automatic winterizing feature. BoatUS: The Boater's Guide to Winterizing (PDF)
Sterndrive and inboard engines
Mercury's MerCruiser sterndrive guide (written for 4.5L and 6.2L MPI models; check your own manual for others) lists:
- Change the oil and filter before storage, because used oil holds acidic combustion byproducts that can harm the engine while it sits.
- Change the sterndrive gear lube and look for water in it. Water can freeze and damage the gearcase. If the old lube looks cloudy or milky, have a technician inspect it.
- Replace the water-separating fuel filter, and on engines with freshwater cooling, check the coolant level and its freeze protection.
- Drain the seawater cooling system, then fill it with propylene glycol mixed to protect the engine down to the lowest temperature it will see. Mercury says it's very important to displace all the water, because water left in the system can freeze and cause serious damage.
- Store the drive tilted down if the boat is kept outdoors, so rain and snow don't collect in it.
Mercury: How to Winterize Your MerCruiser Sterndrive
For inboards and generators, BoatUS's checklist adds: run antifreeze through the raw-water cooling system, make sure the water strainers are filled with antifreeze, fog gasoline engines, and put the batteries on a marine charger. BoatUS: The Boater's Guide to Winterizing (PDF)
Water systems, head and bilge
The BoatUS checklist covers everything below deck:
- Pump out the holding tank and add antifreeze to the head.
- Drain the water heater and bypass it.
- Drain the freshwater system and run antifreeze through it, plus the live well and washdown pump, per your owner's manual.
- Drain the shower sump and anywhere else water pools.
- Clean and dry the bilge, and check that the bilge pump and switch work.
- Remove food, take home portable electronics and cushions (or store cushions on their sides), and close the propane valves and take portable canisters home.
BoatUS: The Boater's Guide to Winterizing (PDF)
Shrink wrap vs indoor storage
BoatUS says the best option is indoor storage in a climate-controlled building, but that for most owners it's either unavailable or unaffordable, so the real choice is storing the boat on land ("on the hard") or leaving it in its slip. Keeping the boat out of the water lowers the risk of several kinds of damage, including sinking. BoatUS: The Boater's Guide to Winterizing (PDF)
If the boat stays outside, BoatUS's points on covers:
- A cover protects against meltwater freezing in fittings and against water pooling on the deck.
- Custom canvas covers are best, on a wood or aluminum frame so air circulates and water doesn't pool, with vents built in to reduce mildew.
- Shrink wrap keeps rain and snow out very well, but traps moisture and causes bad mildew unless it's vented along the whole length. It can also make two-part polyurethane paint peel or bubble where it touches.
- Don't shrink wrap the boat yourself. BoatUS says it's a job for professionals because the wrap can catch fire, sometimes out of sight inside the cover.
- Never tie the cover to jack stands or blocks, because a strong wind can pull them out.
To compare the three kinds of storage space, see covered vs indoor vs outdoor RV and boat storage.
Personal watercraft (jet skis)
PWC have their own steps. Sea-Doo's guide lists: inspect and rinse the jet pump; flush the exhaust through the flush port for about 90 seconds at idle, then blow it out with compressed air at no more than 55 PSI; add stabilizer and fill the tank; change the oil and filter; fog the engine; lubricate cables and linkages; and remove the battery and keep it on a maintainer in a cool, dry place. Sea-Doo says its closed-loop cooling already contains antifreeze and doesn't need flushing, and says to use a breathable cover rather than plastic, which can cause mold. Sea-Doo: How to Winterize Your PWC
Yamaha says to change the oil and filter before storage, charge the battery and bring it indoors if the WaveRunner is stored outside in cold weather, store with the tank 7/8 full of treated fuel, never plug the fuel vent, and leave fogging to a Yamaha technician because some WaveRunner intake systems are complex. Yamaha WaveRunners: Maintenance Matters
For sizes, trailers and where to keep one, see our jet ski and PWC storage guide.
FAQ
Should I fill the fuel tank before winter storage?
On older boats with vented tanks, Mercury says to fill with fresh stabilized fuel to about 95% to limit condensation. Newer tanks that can't vent freely to the air don't need to be filled.
Is it OK to shrink wrap my own boat?
BoatUS advises against it: the wrap can catch fire, sometimes out of sight inside the cover, so it recommends having professionals do it. It also says shrink wrap must be vented along the whole length to avoid mildew.
Does a Sea-Doo need antifreeze for winter?
Sea-Doo says no: its closed-loop cooling system already contains antifreeze. Only the exhaust system needs flushing and draining.
What antifreeze goes in a sterndrive's cooling system?
Mercury says to drain the seawater cooling system and fill it with propylene glycol mixed to protect down to the lowest temperature the engine will see.