Instructables
Picture of Winterizing a Boat with a I/O
There are four things that should be done to a boat to insure it will be ready for the cold temperatures. All these steps are done to rid moisture from the motor and the outdrive so that they do not freeze and crack. The four steps you'll need to complete will be draining the water from the cooling system of the motor, changing out the engines oil and the outdrives oil. The last step would be foging the engine with foging oil. 
 
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Step 1: Draining the cooling system

Picture of Draining the cooling system
Start by locating all the drain plugs located on bottom portions of the motor. there will be anywhere from three to five plugs. Common spots are under each exhaust manifold, on each side of the engine block and on the bottom of the engine block. Today's boats are commonly equipped with either a GM motor or a Volvo motor. Volvo's commonly have a valve that you open and the GM motors have a blue plastic plug with tabs like a wing nut. Weather you have a Volvo or GM motor take caution in the drain valves or plugs when releasing the water. You do not want to break them as this will cause a lot more work. once the water is out of the motor you can close the drains. Put lubricant on each one before closing. Some WD40 for the valves and some pipe dope for the plugs. Once all the drains are closed remove a hose on the top end of the cooling system and fill the system with the pink RV antifreeze. Once the system is full put the hose back on and your done.