Re: acetone
Use Solittle's advise on the mineral spirits first. If it is indeed an oil based stain that was used then the spirits should remove it.
Acetone is very harsh and gelcoat does not like it. Take a clean rag and soak well, apply liberally and let it soak for a minute or so, then try to remove the stain. If you hull hasn't been waxed in awhile then you might need to get a bit more aggresive. An unwaxed gelcoat finish is very porous and it will really suck stuff up, especially thin stain. Better yet, jump on the idiots who didn't cover your boat before they sprayed. Make them clean it.