Non vehicle specific:
Add fuel stabilizer to fuel tank, and run engine 15 minutes or more while the boat is in the water.
Fill fuel tank to full.
Start the engine, and shut the fuel valve off - run it until it dies from lack of fuel.
Flush engine with hose.
Spray a light oil onto all external engine parts that could rust.
Ensure that the engine compartment is completely dry.
In permanent marker on oil tank, specify oil type.
Inspect the jet drive, lube if required.
Inspect the oil injection system.
Inspect all control cables for proper adjustment and lube.
Grease trailer hubs.
Check trailer tire pressure and tread condition.
Check and clean the battery connections.
Place the battery on a "floating" trickle charger.
Hang up the life vests in the garage, so they don't mildew.
Wash the exterior of the boat with the proper type of soap.
Cover the boat with a tarp or OEM cover (preferably in a garbage).
Change engine oil and filter (If its a 4 stroke)
Perform a compression test.
Change all fuel and oil injection filters.
Drain the water out of the exhaust system.
Spray fogging oil into spark plug hole, and then rotate the engine several times.
Rotate the engine while spraying fogging oil directly into the carburetor.
Remove the rear drain plugs from the hull, make sure to tie the drain plugs to the steering wheel.
Inspect the condition, age, and expiration date of the fire extinguishers.
Check the condition of the on-board emergency medical supplies, tools, parts, etc.
That is not a complete list, but a start.