Based on the observation that a filled milk jug will sink, at least in fresh water, I assume that the density of HDPE is higher than that of water.

If you're considering plastic CBs, why not make a simple fiberglass board?  It's bound to be much, much stiffer, and wouldn't be hard to make if the goal was to have a flat, maintenance free board.

Steve Tyree

P-15 #2098, "Amy Ann"