Here's a list of things to do in preparation, based on a diesel auxiliary.
Have shaft alignment checked if not done earlier in season.
Change engine oil and filter.
Change fuel filter.
Change engine zinc.
Check transmission fluid level.
Check lights.
Check VHF.
Check LORAN or GPS.
Make sure reference documents are aboard.
Do a thorough cleaning, especially below decks. A funky boat full of mildew can make a rain-delayed layover or a windless, humid night unpleasant.