Sagittaria, speedwell, scarlet pimpernel,and ranunculus are not invasive, and would look pretty nice on an island.
Sams Club has some nice non woven 20 year UV proof fabric in 4'x200' rolls. I think I'll get a roll and make me an island over a PVC frame. I'll stuff the 4" PVC pipe with styrofoam peanuts and shoot Great Stuff foam into it for flotation.
