Foam will deteriorate under UV exposure, creating nuisance a bit later. Why not use something more permanent, like PVC pipe, say, 4", add a float noodle or foam inside to increase buoyancy, and seal the pipe, making a solid frame. Using either biodegradable twines, or rope netting to create a suspended base. Follow it up with hay/dried grass beds, adding a compacted layer of compost. Add seeds of various sort. Ensure to have the seedlings take roots on land, before introducing into water, since you want the roots to attach to various layers before placing into the water to keep things together.
I wish the DFW department allow me to do various experiments on the local waters, but strict regulations prevented me from doing it. They know my craziness too well.
