Most fibreglass powerboats, and many sailboats, have stringers: long beams running fore and aft that stiffen the bottom and carry the engines. On many boats they're wood wrapped in fibreglass. The glass keeps the wood dry until it doesn't. Then the wood rots inside a skin that still looks sound.
How water gets in
The late surveyor David Pascoe listed the routes: staple holes from construction, screws driven into the stringer without bedding, thin chopped strand mat that water passes through, and boxed stringers with no way for water to drain. His summary of the result: the problem usually isn't discovered until it's too late. A bilge that's often wet makes all of them worse.
How a surveyor checks it
Look first. Discover Boating says brown spots mean water is weeping from the inside out. Look for cracks along the top edge of the glass, glass separating from the hull, and soft spots in the sole above.
Then sound the stringers and press them where you can reach. Around the engine mounts matters most, because the mounts are bolted into the stringer.
Then drill. A small hole through the glass into the wood settles it: Discover Boating says wet or rotten shavings mean a problem. Drill where staining or soundings point, and seal every test hole with epoxy afterwards. Get the owner's consent before drilling a boat you're surveying for a buyer.
What decides the grade
Whether the stringer still does its job. Soft or cracked stringers, or rot under engine mounts, are an A: the engine can move and the bottom can flex. Wet wood in a stringer that's still firm is a B, because it will rot. Exposed dry wood is a C.