Todd:
I'd give some thoughts to closed cell foam, something like the very stiff kneeling pads or exercise mats are made from. There is a local place where I can buy 2"x24"x96" closed cell foam mats that are seconds pretty durn cheap. I placed some in the pond in 18" squares for FHM spawning areas, and after 3 years in the sun they still are in fine shape with no UV protection. They are black in color.
The open cell foam will absorb water and might sink. Look for UV protected stuff too.
