1. Failure to drain the engine block
2. Failure to drain water from sea strainer: Like an engine, the seawater strainer must be winterized or residual water could freeze and rupture the watertight seal. Sometimes you don't know it's damaged until spring launching and water begins to trickle in.
3. Failure to close seacocks: For boats left in the water, leaving seacocks open over the winter is like going on extended vacation without locking the house. If a thru-hull cannot be closed the vessel must be stored ashore - the sole exception are cockpit drains. Heavy snow loads can also force your boat under, allowing water to enter through hulls normally well above the water line.
4. Clogged petcocks: Engine cooling system petcocks clogged by rust or other debris can prevent water from fully draining. If it's plugged, try using a coat hanger to clear the blockage or use the engine's intake hose to flush anti-freeze through the system.
5. Leaving open boats in the water over winter: Boats with large open cockpits or low freeboard can easily go under by accumulated snow. Always store them ashore.
6. Using bimini covers as winter storage covers: A cover that protects the crew from the sun does a lousy job protecting the boat from freezing rain and snow. Unlike a bonafide winter cover, biminis tend to rip apart and age prematurely by the effects of winter weather.
To get a free copy of the BoatU.S. Winterizing Guide full of tips to help you prepare your vessel for the winter, go to BoatUS.com/Seaworthy and click on "Winterizing Your Boat," or call 800-283-2883.