Source : Webster's Revised Unabridged Dictionary (1913)

Water rat \Wa"ter rat`\
   1. (Zo["o]l.)
      (a) The water vole. See under {Vole}.
      (b) The muskrat.
      (c) The beaver rat. See under {Beaver}.

   2. A thief on the water; a pirate.

Source : WordNet®

water rat
     n 1: a person who enjoys being in or on the water [syn: {water
          dog}]
     2: common large Eurasian vole [syn: {water vole}, {Arvicola
        amphibius}]
     3: any of various amphibious rats