PetSmart has plain old monkey grass / mondo grass growing underwater in their plant aquariums. That stuff is tough, so, if it really will grow underwater, maybe the fish will leave it alone.
I have not had good luck with underwater floating plants, ever since:
-I put a pressurized bio filter on the pond; asumedly it removes so much waste that there is none left to feed the plants
-the koi grew big enough to terrorize any plants I put in there.
These days, I have my plants in "corrals" - made of PVC pipe frames covered with plastic sun screen fabric. Allows water to flow through, keeps fish out. I do have small fantails in these containers.