In many areas of the country, you’ll want to winterize a boat before freezing temperatures set in. Even if you use your boat all winter long, there are some systems and items aboard that need some extra attention.

Winterizing A Boat

1. Start by prepping your engine; ensure all water has been drained and removed.
2. Apply corrosion protection to your engine.
3. Consult your owner’s manual for specific instructions, and complete fogging, if needed.
4. Add a fuel-stabilizing additive to your fuel system; then run the engine.
5. Change your engine’s fuel filters and any fuel/water separators in the system.
6. Drain your boat’s freshwater plumbing systems (sinks, tanks, and heads).
7. Add antifreeze to your plumbing systems.
8. Ensure water is removed from all additional systems (raw water washdowns, livewells, bilge pump, etc.).
9. Remove drain plugs.
10. Cover your boat or place it into winter storage.

Source: https://www.discoverboating.com/ownership/storage/winterizing