Tape-grass (Vallisneria Americana) aka Eel Grass or Water Celery
Tape grass in Dolloff Cove - photo by Don Yurewicz
Tape grass has long ribbon-like leaves with a distinct cellar band along the midrib. The leaves are about an inch wide and can be up to several feet long. The leaves have rounded tips. Tape-grass grows in clusters from underground runners, and often forms tall underwater meadows. The upper leaf parts are sometimes found floating across the water surface.
This plant is often confused with the submerged leaves of bur-reed. Tape grass leaves have heavy air-filled, vertically veined centers (lacunae) and lightly veined edges. Unlike bur-reed, this plant has no keel or rib on the back of the leaf. (Excerpts from Aquatic Plants and Algae of New Hampshire's Lakes and Ponds)