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Winterizing the jet boats are fairly easy as they are just an outboard powerhead engine.
First you'll want to add stabilizer to the fuel tank, just follow the directions on the bottle or dump the entire bottle in, it won't hurt anything.
Next hook up your garden hose or flushing device. Remove the front carb covers, turn on the water and start the engine and let it warm up. After it is warm, bump the idle up to about 1500 rpms, then spray fogging oil down the throats of both carbs. The engine will smoke quite a bit, it is supposed to do that. Keep spraying into both carbs until the engine stalls out. With outboard powerheads, it's usually easier to use 2 cans of fogging oil, 1 in each hand.
Thats all you do for the engine. After that you can spray the outside of the engine with anticorrosion spray.
The last thing you will need to do is drain the waterbox's if you have them. Waterbox's are nothing more than water filled mufflers. Typically it's as easy as lifting up the trailers tongue until all the water pours out of the exhaust. If you still hear some sloshing around then just stick a wet/dry vacuum hose to the exhaust outlets to pull out any remaining water.
Service manuals can be obtained from any Mercury/Mercruiser dealer or from www.mercurymarine.com directly.