Sand it!
If the finish is only moderately bad, any good compound and wax and grease-of-elbow <g> will work. but if it's really lousy, wet sanding will make it like new in most cases. The trick is to use a series of grades, each to remove the scuff marks made by the last, coarser grade, For example, start with 220 (or 380 if it's not too bad), gently wet-sand it till the white is back, assuming of course it's a white boat! (if you've gone all the way through the gelcoat and still not white, then what you really needed is a new paint-job). The gel-coat is usually thicker than you expect, and you will usually not come anywhere near penetrating it. Now start resanding with 400, 600, 800, 1000, 1200, and maybe even 1500, which will make it so glossy it'll be like a mirror.You usually won't need to use all these grades of wet-dry sandpaper, because you can skip alternate grades and still get a nice job in most cases. After all that work, and a couple of beers to celebrate, apply a good wax and go sailing!