At one time I was considering this approach myself and I thought that using a half inch thick piece of styrofoam with 3 or 4 plys of 6 oz on each side would be very strong. Again the foam spacer acts as an I beam in the fore to aft direction. By the way, they build airplanes with this technique so I know it is proven and very very strong and very light weight. I saw a demo of a panel made this way and a person went at it with a 2lb sledge hammer and it didn't puncture through.

You could also use the foam spacer with a piece 1/8" ply glued to each side of it and bond this bulk head in place. This would be almost indistructable.

If you use a plywood bulk head and you kick a hole in it or break it lose in some other manner....big deal....its a very easy thing to fix, patch, or replace at any time. My only concern at that point would be the lose of floatation.

Tom