The Collaborative International Dictionary of English v.0.48:

Water radish \Wa"ter rad"ish\ (Bot.)
   A coarse yellow-flowered plant (Nasturtium amphibium)
   related to the water cress and to the horse-radish.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Nasturtium amphibium
    n 1: perennial herb found on streams and riversides throughout
         Europe except extreme north and Mediterranean; sometimes
         placed in genus Nasturtium [syn: great yellowcress,
         Rorippa amphibia, Nasturtium amphibium]