When I bought my boat 2 years ago, it had bad bow stop marks/scuffs and a bad brown water line stain on the hull. I used Megulars #49 oxidation remover for the hull stain. For the cuff marks I first used a Magic Eraser, which only minimally worked.
For the more difficult scuffs I used acitone, and it worked great. Just be careful with it and test an area first.
After this, I use Bass Boat Saver religiously and new marks and water line marks wipe off easily.
Before & After
