Annual Maintenance:
Oil change, oil filter replacement, ATF/power steering flush if fluids are dirty.
Winterization service which includes basic annual maintenance (oil change, complete water drianage from the engine block, etc...)
Adding Sta-Bil to the fuel tank of any boat that is used seasonally, and stored for a season or longer during the year.
Once Every 3 Years:
Replacement of spark plugs, fuel filter, cap and rotor.
Impeller replacement (*** Mercruisers with a plastic water pump housing should be replaced once a year).