The Collaborative International Dictionary of English v.0.48:

Waterwort \Wa"ter*wort`\, n. (Bot.)
   Any plant of the natural order Elatineae, consisting of two
   genera (Elatine, and Bergia), mostly small annual herbs
   growing in the edges of ponds. Some have a peppery or acrid
   taste.
   [1913 Webster]