I think esshup has the right idea. I have planted eel grass, water lilies, etc. using this method. I put the plant and some topsoil in a bag that has been shortened to about 9", covered the soil and roots with sand or fine gravel, stapled the bag shut except where the plant sticks out. Then place it where it was wanted, even in 3' of water. For some, I doubled the bag to make it sturdier when wet. Seems to work.
