FRESH VS SALT WATER. “A number of terrestrial lineages of plants have evolved into aquatic-freshwater hydrophytes and a number of morphological features are shared by both hydrophytes and seagrasses, e.g., the presence of a diffusive boundary layer around the leaves, a photosynthetic epidermis, loss of stomata and the development of aerenchyma.”
http://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-11-8