Sovents won't hurt your boat
Your boat is made of polyethylene which is highly stable to almost all solvents, but be carefull of your outfitting or other materials. I would try in this order alcohol (absolute ethanol, methanol or isopropanol), acetone (might be finger polish remover depending on type), tolulene, or epoxy remover (be careful with these last three, they are very strong solvents). It's possible a non-polar solvent like mineral spirits, kerosene or gasoline might do the trick. I would recomend wearing polyethylene gloves or you could use vegtable bags from the store to protect your hands; and not breathing the fumes.