is similar to rubber cement and plain naphtha or benzene will release it. But you can also buy a spray can of "vinyl decal remover" at most auto parts
stores (go for the brand name) and those usually have solvents which will be faster, as well as something to help prevent the plastic from breaking up as you try to lift
it. About $5.
The heat gun helps, pros use heat and solvent together--but make sure you don't start a fire that way if you try to. Especially with acetone or higher end solvents.
You may find that after you've lifted the vinyl, there's a "brighter white" shadow under it, and the rest of the hull
has faded or chalked so there's still a ghost image. If that happens, try cleaning
and polishing the area. If the shadow remains, all you can do is cover it entirely, i.e. with a background sheet behind your new graphics. (Well, that or repainting the entire transom.)