"Fiberglass" boats are in most cases anything but. To get strength at a minimum of weight and materials, the structural portion of the vessel will be built out of another material, often wood, working ultimately as a composite structure. You will have a stringer section, that is likely of wood or foam, either bare wood or wood covered in fiberglass. Your transom is almost surely cored with wood. Your decks, again cored with wood or foam. Any of these cored areas, if not 100% sealed, and not just the day it was built over 20 years ago, but also every day since then, all have the potential to eventually allow water intrusion. Once that happens, it can be very difficult to dry. In fact, once water logged it is in fact the drying process that tends to create rot, the wood stays happily wet and intact before that.
Tapping the hull is a simple, but effective method, often used by surveyors. The results from a thorough tapping from a learned individual are usually more effective than a moisture meter, the interpretaion of results from which is almost entirely in the hand of the user, not the machine itself. Essentially, one uses a non marking plastic headed hammer to "tap" the entire hull, looking for unexpected changes in the resulting tone. You calibrate the tap by establishing a benchmark sound in a known condition area and then listen for changes, usually a "dull" sound whereas other areas produce a "ringing" which indicates a change in the core material underneath (water intrusion).
Another method is to use a drill bit and drill into the core from an unseen location on the inside of the boat (not drilling all the way through). If the core is dry, you simply fill the hole with resin filler and move on. The drillbit technique is very effective, just be sure to fill any holes and you also must know your boat well enough to know where all coring areas are located on the boat.
Old compound foams and those in contact with water over many years can easily absorb water. Take a hard look at any "flotation" foam as it definately has the potential to turn into a gigantic sponge. An electric knife is a good tool for taking core samples of flotation foam, again seal any area that you expose.
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