The foam blocks in our boats (or mine, at least) that I have seen are expanded polystyrene (or EPS, as opposed to extruded polystyrene, or XPS). When submerged these will become waterlogged at a fairly high rate (weeks, or months), but, because they are located high in the hull, and not in the "bilge" or lower areas of the boat, the risk of them ever becoming waterlogged is very low.
I can’t recall ever seeing any polyurethane foam-filled spaces – anyone else seen any?
(Ply)wood has not been used in the lower areas where water would collect- it’s all fiberglass in the areas I’ve looked.
Someone on a thread in the last year here reported seeing foam inside the ballast tank. Being curious, I placed and inspection scope into the ballast drain port (insert your own proctologist joke here

) and did indeed see something that looked like exposed foam.
But on closer inspection I determined it was actually a foam-coloured and textured fiberglass structure (stringer? bulkhead?) for some reason of a different colour polyester resin than was used in the rest of the hull. Or possibly enclosing foam or something foam-coloured. (I did't need to snip any polyps - owwww

)
A factory tour, on “the list”, will solve a lot of mysteries.
-B.
