I was lucky in that except for the 'skier girl', my old decals were already missing! I could still see the outline of where they used to be in the gel coat. This helped when installing them. I waxed the entire boat with 3m wax, then used rubbing alcohol where the decals went. After that I just followed the directions that Ron gave me.
I don't envy you having to remove the old decals! I've used Goo Gone before, not a very big fan of it. When I remove decals from other stuff, Wd-40 or Pb blaster seem to work best, I would just be worried about using those products on gel coat and/or clear coat on the boat. I'm sure others can give you some pointers to make it easier.
-Matt