: I guess I would tend towards a more water resistant material, but
: CARDBOARD ?! - just might make me be sure to get it right and
: waterTIGHT (-:

Yes, I agree. Why not thin balsa ply? Would be stronger and I bet it wouldn't add too much weight. Right now he's about 2 pounds lighter than a conventional foam board, but I think with balsa if he added that 2 pounds back still he would be way stronger than any foam board, probably equal to a wood board, except for puncture resistance.

This does bring to mind an idea for someone wanting to build a really light surfboard. You could just swiss-cheese a foam blank with tiny holes before 'glassing it. I would just put three balsa stringers instead of one, with the other two being spaced out from the center stringer. That should give some strength.