How long does the plywood sheets float before it sinks? Is this marine grade plywood or standard OSB, or chipboard or what?

My pallets became waterlogged and heavy as lead and sunk after one season in the water. I love your idea of keeping them in place with guide ropes but I would have to figure in some way to keep them on top.

I'm curious if the FHM would prefer wood surface over foamboard? It would seem logical to use high density foam sheets since it would take much longer for them to waterlog? Or maybe plastic sheets?

I'm getting ready to build a forage pond this year and really want to figure out the optimal substrate that FHM prefer.