Probably like all other delams: Water left inside the boat+ heat + sunshine = foam becomes brown crumbles between the skins.
1. Mark then Cut the skin off with a Skilsaw set to 1/16 of an inch or just a bit more.
2. Peel it up to see what you find and clean up the mess. 3. If wet brown sawdust crumbles... decide to fill with canned polyU foam which is difficult and messy to sand to get flat. But is possible, then replace deck skin.
4. Or clean up and fill with hard polyurethane 3/16 inch flat stock surfboard foam, sand and replace skin= clean job.
5. Or fill hole with glass fiber mat and microballoons before putting the skin back on= heavy repair.
6. WORST finding- If the underskin (below the foam) is all shot and hanging down, this is usually when Hobie 16 and 18 hulls are sawed up and dumped. Building this up is heavy and ends up ugly but can be done, is there is a hull worth saving.