Wikipedia
Cymodoceaceae
Cymodoceaceae is a family of flowering plants, sometimes known as the "manatee-grass family", which includes only marine species.
The 2003 APG II system (unchanged from the 1998 APG system) does recognize Cymodoceaceae and places it in the order Alismatales, in the clade monocots (see also the 2016 APG IV system). The family includes five genera, totalling 17 species occurring in tropical seas and oceans (so-called seagrasses).
According to the AP-Website it is doubtful if the family Ruppiaceae is distinct enough to be kept apart: the plants in the three families Cymodoceaceae, Posidoniaceae and Ruppiaceae form a monophyletic group.