If you decide to go route, do it very gently.

Gently? Try no rubbing at all. Just put a paste of it on there and let the
acid do the work. If you rub it you'll just be putting more scratches into
the gelcoat. Making it easier for the gunk to attach to it again next
season.

Your best bet is to stay on top of this during the season. Just use a
soft/medium brush on it. Shur-hold makes an attachment with bend in it that
makes this less hassle. If you're in the water you can use one of those
suction cup handles to give you a hand hold to really get a good scrub.

-Bill Kearney