Coir is a good option and it is organic. Paul Kay of Rogue Water came up with something he calls a "botanical burrito". You can roll some inert media like lava rock or expanded clay in a bit of coir (not too dense of coir for this application) and
maybe a wee bit of compost (i agree with posters that the nutrients in the water should be sufficient for growth, but a head start wouldn't hurt the plants...still, you don't want to add nutrients to the water if they are a pollutant that you'd like to remove from the water).
Roll about four of these up like large burritos and poke holes in their tops to insert bare-root native sedges, rushes, or whatever wetland plants you like. Then lay about four "burritos" in a standard nursery flat. (Be sure they get saturated when deployed!) Paul floats these in a PVC frame that provides adequate flotation. (He says that it is important to get the dimensions of the frame very exact to hold the flats securely.) Other frame and floatation options could be considered. As an experiment, I successfully grew some plants in a burlap sack filled with nothing but dried blackberry canes and empty water bottles (almost free material) that I floated in a creek.
You might look at
north fork natives, wetland sod to get an idea about how little substrate wetland plants need to thrive.
I know I haven't added much about building the floating structure (many experiments swirl in my head!) but hopefully these materials and methods help stimulate some ideas...good luck!
