I had something similar on a car, tried everything to try to remove it. Took it to a paint shop and the spots were acid rain. Needed to have the vehicle repainted. I hope you don't have acid rain, mine was garage kept and always kept clean with wax, but no wax or cleaner would touch it. It would always appear to have water spots that were only visible under lights. The gel coat may react different. You might want to get a detailer to take a look at it. If it is acid rain it just won't come off. Good luck
Chuck " still in Va "
2012 3065w Windjammer
2012 f250 lariat Crew cab power stroke (of course )