Grind, grind, grind
I purchased a boat once where the owner had tightly wrapped the boat in plastic over a too long period of time and the result was moisture captured within the cover and this resulted in blisters, just like your problem. The good thing about it is we renegotiated the price down but the bad thing is I had to fix the blisters. That meant drinding out all the bad gelcoat and fiberglassing it up and then fairing it smooth with putty and then, since it was too much to touch up the gelcoat, I repainted the whole hull with Awlgrip. Looks good now, but it is a lot of work. Bummer for you. Have fun.
