The Collaborative International Dictionary of English v.0.48:

Waterweed \Wa"ter*weed`\, n. (Bot.)
   See Anacharis.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Anacharis \An*ach"a*ris\, n. [NL., fr. Gr. 'ana` up + xa`ris
   grace.] (Bot.)
   A fresh-water weed of the frog's-bit family
   (Hydrocharidaceae), native to America. Transferred to
   England it became an obstruction to navigation. Called also
   waterweed and water thyme.
   [1913 Webster] Anachoret

WordNet (r) 3.0 (2006):

waterweed
    n 1: a weedy aquatic plant of genus Elodea