As with the other questions concerning blistering, this may indicate a very serious problem that will only get worse.
Depending on the age , water temperature and construction of your vessel, water by capillary action, inters through the gelcoat and spreads into the fibers of the glass. It forms a caustic acid and eventually will weaken the structure. You will need to investigate the situation.
Remedies may be as simple as opening each blister and fill with a ferring compound. or as complicaterd as having the entire affected hull power planed and barrier coated and gel reapplied.
My advice is to repair it sooner than later.
Good luck JEF