What to do depends on the size and amount of blisters.
If you want it done right (and want to do it yourself), sand the blisters and the gel down to the glass. WEAR GOGGLES. When you sand the blisters, they might pop and squirt stuff that you don't want in your eyes. Keep sanding until you get "good glass," if there is a void. Good glass is saturated with resin, not "dry" like a fiberglass cloth. Glass over it with high quality resin and re-gelcoat it with "good stuff."
If you don't want to do it yourself, disregard above and bring it to a glass shop.
