Fiberglass boat hulls are subject to the formation of blisters within the fiberglass hull layers below the waterline. These blisters vary in size, but all represent a damage site to your hull. The blisters are a result of moisture being absorbed into the fiberglass, probably due to sites of less then perfectly cured fiberglass polyester resins. A smelly, watery fluid fills the blisters and may actually squirt out when the blister is punctured. Repair and prevention are topics of much debate.
Repairs range from the complex to the simple.
One end of the spectrum consists of sealing the boat in an airtight bag and using commercial dehumidifiers to suck out all moisture in the hull followed by completely cutting out the blister and repairing as one would repair a hole in the hull. Many people will let a hull dry out for weeks after lancing and opening a blister. On the other end of the spectrum, some people will do little more than lance a blister and almost immediately putty over it. Prevention currently consists of applying several thin barrier coats of epoxy to the hull. Epoxies are less porous than the polyester resins used in production fiberglass boats.