I'm with TPF also. I was going to say Hydrilla, but I'm not 100% sure and I know that it's hard to kill.

I have the same grass around my pond, and it makes good shoreline cover. My pond water lever fluctuates, and that grass will grow thru water, which for me is a good thing.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).