When I had my boat, I used Collinite fiberglass heavy duty cleaner (920), their marine wax (925), then their high gloss sealant (925). If I had a lot of slime, green, rust stains or brown crap on the hull or along the sides, I would use On-Off FIRST, be careful it's ACID, DO NOT SMELL IT, I did and almost passed out, it will not hurt the gelcoat. I suggested these products to my other boating friends, they liked it also. Again, it's just my lousy opinion.
G