Roll off work on boat surface mold/mildew grimy areas. Mostly deck and rubrails from being stored outside with tarps over etc.
some have said diluted bleach mixed with a little dawn dish soap works but never used bleach on gelcoat.

rolloff is good chit,we use it on the sea ray all the time.

Roll off works better than almost anything else I've ever tried. What I would do is give my dad a call and and take the hot powerwasher to it. It will be clean in 30 minutes vs a full day of detailing. I'll touch base with him later today for you.