Tough to tell - opposite leaves is a big clue - omits some pondweed genus - (pondweed is a very general name for many genus and even families). I found a website - http://www.chesapeakebay.net/bfg_widgeon_grass.aspx?menuitem=14363 either Ruppia maritime, Widgeon Grass or Zannichellia palustris Horned Pondweed. Sago pondweed is alternated leaved along with Potamogeton genus so they are omitted. Hopeful someone else could help you accurate id it.
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