I can't be sure from the photo, but I think mphunk is correct; it is likely Crinum thaianum, or water onion. Does it have a small onion-like bulb at its base? That is one sure way to tell. If not, it is most likely some variety of Sagittaria or Vallisneria, which can be easily differentiated from Crinum spp. because they will put out runners sooner or later.CoralK:
Could that be a water onion? It's more of a pond plant, if it is...
OK, I'll quit rambling