There's a thread buried in the forum about removing hull scratches that was written by Cal, our resident VilageIdiot.
Overall, I'd personally suggest ruuning either the 3M or StarBrite lines to find the least-destructive combination that would work. And you've got to use a boat soap, because car soaps and even dish soap have agents that aren't too gelcoat-friendly. Read back through the owner's manual, too.
I bought some boat soap, 3M cleaner/restorer, rubbing compund, etc. this spring but spent so much time on the vinyl that I didn't get to the gelcoat.
And maybe buy some non-marking fenders? Either way, in that kind of environment, it's probably very important to keep a good coat of wax on the hull so that the yucky water eats wax instead of gelcoat.