Water lillys are apparently posionious but the fish do fine with them. Snails will take the odd bite out of a Lilly then leave it alone. I would experiment with plants that are non toxic and safe, or known aquatic plants, just to be sure. If the mint is like my aquatic mint, it will jump out of the pot and throw shoots everywhere. My aquatic mint is not potted, but it does grab on to the nearest plant and run loose from there.
I find most large aquatic plants with large roots do best when not potted, and allowed to roam free in the pond, My large sweet flag is growing quite nice too, although it does not get as big or quite as tall the my yellow flag. I have often just taken shoots from these plants and shoved them in a shallow area, wedged in with rocks, no soil, and they grow best that way. If you put these large plants in a pot, they jump out or their growth gets stunted from being contained in soil.
Keith I think you are right, lots of plants have adapted to water over time. In the wild pond, the pond can some years go dry but lots the plants will still survive, and sometimes areas that were dry turn into wetlands, and again, grass and other plants will adapt and there will always be plants growing at the waters edge....
Mtpond, your yellow flag will grow, just takes time. If it looks like the pot is to small, then it is most likely getting root bound. You can take a shoot from the pot and start one in the shallow rocks for now, with no soil, and that shoot will grow quick without any container to stunt it's growth.
