Al, I like Elodea and it may work well because the summer heat would probably check its growth. However, when ever you plant a weed, you need to be prepared to deal with it if it grows beyond what you think is acceptible.
Other great plants that you can consider in Texas include:
Eel Grass or Wild Celery (Vallisneria americana)
American Water Plantain (Alisma subcordatum)
Spike Rush (Eleocharis spp.)
Rushes (Juncus spp.)
Bulrushes (Scirpus spp.)
Sweet Flag (Acorus calamus)
Iris (Iris spp.)
Good luck!
