SEAGRASSES. “The monocotyledonous seagrasses represent, in fact, a polyphyletic group of plants that can live underwater in fully marine environments. At least three independent seagrass lineages, but no other angiosperm species, have evolved to a life in the marine environment.”
http://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-11-8