I would bet a coke it is water primrose. I have some but it stays in check. Lots of duckweed.
Try the below site, it has pictures and control methods.
Once my water primerose was getting out of hand and I just waded and swam around and gathered it. You won't get it all, but you will slow it down. I kind of like it in my pond. It will make a good ground cover as the pond drops. And next spring will make a good cover as it is flooded and grows upward to the top of the water. It usually only will root in pretty shallow water. And the roots are easy to pull up. It is the broke off pieces that keep it coming back.
Ahhh an edit because I didnt notice Chris' post, I should refresh more often.
http://aquaplant.tamu.edu/
