Looking at all the photos, it's not watercress and it isn't hydrilla.
What's confusing is that it's immature. I've not seen that plant (that I know of), but my gut response it that it's a species of pondweed. If it's variable-leaf pondweed, it's not through maturing, yet. It's got all the earmarks of one of the pondweed species, but the leaves are throwing me off.
