I sorted out the link to be able to get direct access to it. I cannot identify it positively without seeing a better closeup picture.

http://img125.exs.cx/img125/3874/Image80.jpg

Like Kelly said in her post it will be very important to know more detail about your situation, location, etc...

Here are some online resources that may help.

OK
http://www.apms.org/plants/plants.htm

Better
http://www.ppws.vt.edu/scott/weed_id/aquatics.htm

Good Luck,

J.W.