I agree with John that some plants won't do well when their roots are just in water, but as Bill said there are certain species that will thrive in a hydroponic enviroment.
The other factor in selecting plants that will survive on your island is your climate, all the different water loving species I planted the first year grew well but only the swamp grass, water Canna, and Iris survived the winter with the water Canna not surviving the second winter.
The Cattail planted itself and survives along with the Iris and swamp grass. I'm sure there are many more plants that will survive such as the pickerel weed which I have on my natural islands but never planted them on the artificial island.
