The Collaborative International Dictionary of English v.0.48:

Water grass \Water grass\
   (a) A tall march perennial grass (Paspalum dilatatum) of
       the southern United States and the American tropics.
   (b) Manna grass.
   (c) The grass Chloris elegans.
   (d) [Dial. Eng.] (1) Velvet grass. (2) The water cress. (3)
       One of various horsetails.
       [Webster 1913 Suppl.]