Looks like foam...

I found a bunch of Divinycell H80 foam sheets locally and we are thinking of going with it for all the interior parts, deck and bulkheads. The designer, Jaques Mertens has forwarded the scantlings for its use and it looks very doable. Of significance here is the speed at which you are able to get information from the designer. I posted my questions on the forum and when I checked back in fewer than 30 minutes he had posted the response and emailed me the scantlings. More to come as we are really on our way now.