Source : Webster's Revised Unabridged Dictionary (1913)

Water hen \Wa"ter hen`\
   1. (Zo["o]l.) Any gallinule.

   2. (Zo["o]l.) The common American coot.

Source : WordNet®

water hen
     n 1: North American coot [syn: {American coot}, {marsh hen}, {mud
          hen}, {Fulica americana}]
     2: any of various small aquatic birds of the genus Gallinula
        distinguished from rails by a frontal shield and a
        resemblance to domestic hens [syn: {gallinule}, {marsh hen},
         {swamphen}]