Do you know what your boat is made of? If not, a core sample is a sure way to find out. Some manufacturers keep the pieces they have cut out for thru hulls, underwater hardware, vents, etc. to be able to show the customer the quality of the boat they built. Core samples are also pulled in surveys to check a spongy area for rotten core and resulting delamination.
In the pictures above, the core sample on the left is made of many fiberglass layers whereas the sample on the right has closed cell core sandwiched in between layers of fiberglass. The sample on the right is what helps make the boat light and buoyant. The sample on the left is more reinforced. This sample was pulled from the Patriot II where the rudder goes into the boat, an area that needs to be stronger.
Many older boats have balsa (wood) core as opposed to closed cell plastic core. The pictures above show a common complication of wood core - Rot. The picture on the left shows rotten core between two layers of gel coat and glass. The picture on the right was taken in the process of removing the rotten core to replace with closed cell core. Knowing the structure of your boat is key. Water intrusion is the culprit in all rot. If you have a boat with wood core, you must be sure to follow appropriate installation procedures to avoid rot and subsequent delamination.