Originally Posted by JimN
Any time something is on a surface and it doesn't come off by wiping, you need to find out what the substance's base is- water, acid or oil. If it doesn't come off using water, use vinegar. try denatured alcohol, then mineral spirits. If that doesn't work, use Oops or Goof-Off. You always want to use the mildest solvent on gel coat because it's porous and if you use a really aggressive solvent, the staining can become permanent.
If the hull was waxed before you went, it's likely that these spots are oil or ammonia-based and those will remove wax. Ammonia is one of the by-products of decaying animals and vegetation and WRT Ph level, it's somewhat basic, while waxes and oils are acidic and a solvent with the opposite range of Ph will dissolve it (acids and bases neutralize each other). If an acid won't remove the stains, try a solution of baking soda and water.
well said Jim, one of the reasons I wax often even the hull... I use a mix of 5% white vinegar and 5% car wash when I get the boat home... all the spots come off. .. I know this is tough for people who keep their boat in the water in a slip. When I go to Canada and I leave the boat in the water for a week or more I put about 2 coats of Carnuba on the hull.
Seems to work when I pull and wash hull and everything comes off easily.
I bet the water makes a big difference as JimN says.