i use rum....and sometimes i even clean my cushions...
but seriously a good dunk in a well clorinated swimming pool then a wipe down with a good cloth cleaner, rinse well with the garden hose,and put inside you home with the fan on them for about two weeks. make sure they are good and dry...then use more rum...
now go about finding out why the inside of your boat is so wet that your cushions are molded......that is the real problem i suspect
good luck
keith