A fibreglass laminate is layers of glass bonded with resin. Many decks, and some hulls and transoms, are cored: two thin skins with balsa or foam between them, which makes a stiff panel for little weight. Delamination is the bond letting go, between layers of glass or between a skin and its core.
Once the skins part from the core, the panel loses most of its stiffness. A deck goes springy under foot. A transom flexes under the outboard. Water in a wood core is the usual cause, and it spreads.
What a surveyor checks
Sounding with a phenolic hammer, in a grid. Soundings describes the tell: a deep, hollow note, and the hammer doesn't bounce back as strongly. A moisture meter maps any wet area against dry parts of the same panel, read as a comparison, since Practical Sailor found meters can't give a true percentage in fibreglass. Walking the deck for soft spots, and looking for cracks and weeping round fittings.
In the report
Give the area's size and position, the sound, the readings and whether it flexes. "Foredeck, area about 600 by 400 mm round the windlass, dull on sounding, high meter readings, deck soft under foot." See wet deck core.