Most yacht decks, and many hulls, are a sandwich: two thin skins of fibreglass with a core between them. The core is usually end-grain balsa, foam or plywood. It makes the panel stiff and light while it stays dry.
Water gets in where fittings are bolted through and the sealant has failed. It spreads through the core, and balsa and plywood rot. The skins part from the core and the panel goes soft.
What a surveyor checks
Walk the deck for soft patches, usually round stanchions, chainplates, the windlass and the mast. Sound it with a phenolic hammer: a sharp note over sound core, a dull one where it's let go. Then map it with a moisture meter, reading against a dry area. Practical Sailor's tests show a meter reads only about a quarter of an inch deep, so treat it as a comparison. Last, load each fitting by hand. One that moves means the core round it has gone.
In the report
"Foredeck, starboard side round the forward stanchion base, about 300 mm across: dull when sounded, readings well above the dry reference, stanchion base moves under hand load. Wet core. Remove the fitting, cut out and replace the core, rebed." See wet deck core.