Inspecting a boat for problems within the fiberglass structure is very important.
Rotten wood is common and disbonded and delaminated layers are a big problem.
There are several things to consider and look for.
The video below demonstrates that sound is an important tool to help identify problems.
From Skipper Rudy
Fiberglass boats – testing your stringers for rot.
Also explaining the differences of; bond, dis-bond, de-lamination.