[Edit: I have since discovered that the peculiar structure on the stem is most likely a gall of Membranosorus heterantherae for which I have made a separate observation https://inaturalist.ca/observations/34872865. See also https://inaturalist.ca/observations/32170005]

Whole plant washed ashore. Photos of stipules [actually ligules] stem cross-sections, leaf venation, rhizome (?). This is a common macrophyte at this site in permanent water ca. 1-2 m deep over muck. Around the same size or slightly smaller than flat-stemmed pondweed but lighter green and stems less compressed. Largest leaves on this specimen 4.5mm wide but most about 2.5 or 3 mm. Lacks floating leaves and nodal glands.