Check the bottom and prop conditions.
Check the inlet strainers and scoops for the engines.
Check the battery condition and check the water levels and sp. gravity in the batteries.
Change all the fluids, oil, transmission, antifreeze, filters spark plugs and belts. Have spare fluids and plugs on board.
Check the packing glands, including the rudder packing.
Check all of the bilge pumps to make sure they pump, good. Think about a manual pump also.
Clean the spark arresters.
Fill the tank with good gas and a fuel stabilizer(just in case). Bring plenty of fuel filters.
Have a good tool set on the boat.
Have plenty of sunblock and sun glasses on board.
Bring good charts and guides.
Have a good GPS and good binoculars. Check wiper blades.
Make sure all the safety equipment is in working order and not expired. Including a good first aid kit.
Be sure to have another knowledgeable and able bodied boater on board.
Pack bumpers and lines.
Pack lots of water.
Have a good anchor, chain, rode plus a spare anchor, chain and rode. Just in case.
Just a suggestion, have some different sized wooden plugs to be able to plug off any holes whose hoses may come off(ex. exhaust hoses).
Get a good survey done on boat AND engines before leaving.