If the blisters cover most of the hull below the waterline, it's called "pox" and is a relatively big job to fix. At a minimum, the gel coat and possibly some of the layup
has to be stripped off, using a special tool, and that's the easy part. Then the hull has to be dried out and the stripped area must be recovered either with new layup and/or gelcoat
, or a barrier coat.
If there are relatively few blisters and they have to be repaired, it's not a difficult repair, just somewhat time consuming. But it can be easily do-it-yourself.
On edit: blisters are much more likely with boats that spend a lot of time in relatively warm water
. Watch out for those Florida/Carib boats!!