A very interesting vehicle - a true "eggshell with a hammer", as it were. And there's no doubt another reason they were so "empty" inside: they had to float. Particularly after with all those fully-loaded troops in there, they'd have to have some significant airspace, to hold it all above water!
Speaking of which, I do see and appreciate your "wetted" areas - very nice touch. And a very nice submission to ModelFleet, sir!