Fully submerged macrophyte, common in the lagoon, with semitransparent, uni-nerve and emarginated leaves. Fruits still absent, but alternative species C. lusitanica has a different habitat.
Other pics here
https://www.inaturalist.org/observations/200001133
https://www.inaturalist.org/observations/199682165
https://www.inaturalist.org/observations/199708451
On a small recently dried pond/moist sandy area