It's normal practice to scrape off antifouling paint as part of a survey - don't remove any thing else though!
Osmosis is all about difference in contrations of solutions either side of a semi permeable menbrane.
The resin and gelcoat is the membrane.
The liquid in the blister site is the concentrated solution.
The water is the weaker solution.
The blister forms and enlarges because the water will migrate into the concentrated solution until the solutions are equal in concentration. This never happens though.
Fresh water is a "weaker" solution than salt water. Hence a greater difference in pressures driving the blistering process.