I'd have to call and ask on the plywood type, but never seen one sink. The cables are taut but not overly so..
I had 13 oak pallets on my pond, group of 7 and 6 from mid july to just before I sampled in October with no sinkers. Must depend on wood and pallet design because I've had people tell me they've had 1 or 2 in a group sink. That's where I'd shove a noodle on each side.
As for foam, if you're not exposed to high winds, maybe that would be ok??
I know they've tried a lot of different materials at the Pratt Hatchery and they settled on the plywood.
I'll call the bossman and find out the type of plywood.
I guess they are using 1X10, 4-6' long pine now...
I'm told the AC Plywood WAS coming apart prematurely..