Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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