Winterize your boat!
Fuel -Fill your fuel tank to avoid a build up of condensation over the winter months. Adding a fuel stabilizer will keep your fuel from losing octane and varnishing in the tank. Fuel Stabilizer also fights condensation and water. A good stabilizer will help keep water from separating to the bottom of the tank kind of like oil and vinegar.
Engine-You should run the engine to warm it up and change the oil while it is warm. This tends to allow impurities to be drained away with the oil. You should also change the oil filter. Flush the engine with fresh water. You should drain the engine block and exhaust manifolds to stop corrosion to the inside of the cast iron block and cast exhaust manifolds. Most people think that they need to drain the block because of freezing temperatures. Very rare do we in California at sea level have to worry about freezing and cracking the block. It protects the engine from rusting and flaking off the cast iron and clogging water passages. While you're in the engine room you should also change the fluid in your transmission(ski boats). Spray "fogging oil" into the engine carburetor to coat all internal parts with a "sticky" film to keep them from rusting during the off season. Everything from the intake manifold to the exhaust outlets are coated to fight rust!
Now is the time to address any mechanical problems that you might suspect.
For example, if you here a slight tapping noise and the engine really doesn't idle thats great but if you ignore this problem till spring, your engine may seize up from
rusty water that has leaked into the cylinders!
See Photos below!! Click to enlarge!
Most boats Change the oil and filter. Add fuel stabilizer. Run engine to distribute stabilizer to Carb or Injectors and fog engines internal parts. Drain all engine and exhaust water cavities. Including and/or heaters and showers etc. Disconnect the battery or shut off the battery switch.
Engine that has never been winterized. Note clogged water passages around cylinders. (Below)Water in cylinder from condensation.
Below is a engine with clean water passages and no rust on cylinders