A fibreglass hull isn't fully waterproof. Over years afloat, water passes slowly through the gelcoat and reacts with soluble material in the laminate behind it. The fluid that forms draws in more water, pressure builds, and blisters rise on the underwater hull. That's osmosis, or osmotic blistering.
A true osmotic blister, opened, usually releases a fluid with a sharp, acidic smell. Blisters that are dry inside and sit in the antifouling are usually paint blisters, which matter far less.
What a surveyor checks
The underwater hull on the haul-out, once it's dried. A moisture meter across the bottom, compared with dry areas above the waterline. A phenolic hammer for hollow spots. A few blisters opened to see how deep they go. Practical Sailor's tests found meters read only about a quarter of an inch deep, so readings guide the surveyor rather than measure the damage.
In the report
Say where the blisters are, how big, how many, how deep and what the meter showed. "Underwater hull, scattered blisters 5 to 15 mm across, fluid-filled, confined to the gelcoat. Elevated meter readings against the topsides." See osmotic blistering.