It is caused by a process known as osmosis. Water is attracted through the gelcoat to reach unsaturated resins in the fiberglass. Fiberglass is not completely waterproof. Best vinyl-ester resins still will allow this to some extent. Most common place for blisters is where the hull sits on wet bunks for long periods of time. Even more common on larger boats that stay in the water 24/7. Neighbor had these in his ice chest in a brand new Ranger back in the spring. Forgot to remove drain and let melted ice sit for a couple of months during the winter. Looked very similar to that.
for bunks, the problem is a tough one since carpet on top of non-porous wood against hull makes it very difficult for the moisture to migrate out of the carpet, so it tends to migrate through the gelcoat instead. One of these days, we might see a material like the gatorback stuff that works with BCB hull design. If you are anal, you can always jack boat up off trailer bunks when it is stored, but that is a PITA. I have been using a strong fan blowing down the bottom of the hull and so far, that seems to be working, but I only have 3 years worth of data so far.
For other places, such as top of deck and such, simply keep it dry. 2 days of wet won’t hurt if you are fishing in the rain. But don’t give it two weeks or two months of sitting there with standing water trapped.
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