Sunfish have a closed hull with which includes Styrofoam type blocks secured in place with expanding foam. Everything is sealed at the factory.
The Sunfish hull has 4 major fiberglass parts: The hull, deck, tub and dagger board trunk. When the hull and deck are assembled they insert large foam blocks which are glued in place with expanding foam. This assembly becomes a sort of foam sandwich, with the blocks acting as stiffeners for the hull and deck. If they break loose the hull and deck will flex causing the boat to slow down in waves. In extreme cases it can weaken the whole structure. source
On my older sunfish and many others the only drain is on the top deck near the cockpit (not to be confused with the bailer in the cockpit). To drain the internal compartment you remove the plug and tip the boat on its side. It is not unusual to pickup a sunfish and find there is water sloshing around inside the hull that has not been fully drained. In some cases even boats without sloshing water will have absorbed enought water to have gained significant weight.
If one of the foam blocks is loose or needs to be temporarly moved to make a repair; First you need to install a deckplate to access the internal area, when you are ready to restore the foam block, use expanding foam to lock it back in place.
The problem is the habbit of sunfish hulls to collect and hold water (some years like 71-72 are extremely subject to this). Expanding foam comes in two types; "Closed Cell" & "Open Cell"
Closed Cell Systems are water-resistant, but not waterproof. Minimal exposure to moisture will not affect these foams, however the foam may be compromised if submerged under water for a period of time. Conversely, the Open Cell Systems will absorb and retain water if exposed. Therefore the Open Cell Systems should not be used in areas where moisture may be present. Source
The terms; marine Foam, waterresistant foam, waterproof foam, watershedding foam, etc. are used through out product descriptions found online.
It seems obvious at this point that closed cell foam (of which stroyfoam is one) is the best choice. But there is a lot of grey area in statements like "may be compromised if submerged under water for a period of time".
What do I look for to find an expandable foam that has waterproof capablities simular to the Styrofoam type block?