I just stumbled onto your island making and I gotta have one. A couple of things. Don't use the white beaded foam as it will soak up water and is open celled. Use the blue or pink. They are closed cell and won't soak up water. Just to test the real hardiness of the blue foam,,,, last July I took a square foot of it, heat melted the outside top and sides with a heat gun and that makes a tough outer plastic skin,,, and tossed it into the pond. It is setting in the pond now and is just fine. One other things comes to mind. There is a rope material that draws water up to the plants. I could see punching holes through the plastic and threading the rope through them into the soil and letting just a little bit dangle into the water to keep the plants moist. A good growing medium is a mixture of equal parts of potting soil, perlite and vermiculite. That would keep the growing medium lighter then just soil alone and the perlite keeps the soil airy and the vermiculite keep moisture in the soil. I really like your idea.
When I was boat building I punched clean holes into blue foam by taking copper tubing and twisting it with hand force through the foam.
