As an amateur boat builder I can attest to the fantastic properties of expoxy as a glue and a sealer, but I can also attest to the zelous nature of water to get where it shouldn't and water just loves a "sealed" box. Again I don't really think it's going to amount to much. Riks web page shows a very well made floor and I doubt he will ever have any trouble, it's more of an academic question at this point.
On the question of how to protect the other sealed spaces such as walls and roof I intend to paint all the interior surfaces before assembly to help protect the wood from any water that makes it in. Sort of like painting the bilge or frames in red lead on a boat. Epoxy or CPES would be great but too much money for that application IMO.
My approach to the water problem is a fatalistic one. I know the water will get in someplace so instead of trying to fight it off and seal it out completely, leaving me helpless once it gets in, I'd rather open the floor space up, let the water hit the foam and the bottom of my ply floor. The sealent will protect the wood itself and with no box to infiltrate I don't have to worry about it getting in anything. It's the zen approach to TD floors
"so if sealing the ply works why wont sealing the outter layer on the sandwich work to keep it out of there"? Good question, beacuse there are no joints to fail or fasteners to funell water through. The ply will take some infiltration even throgh roofing tar but it wont be trapped in a open space where it can mix with the air and brew up some nasty wood eaters. IT will probabyl just cook out of the ply as it dries. This is how my plywood canoe works right now (no epoxy sealer just good ole house paint on ply).