The Collaborative International Dictionary of English v.0.48:

Muskrat \Musk"rat`\, n.
   1. (Zool.) A North American aquatic fur-bearing rodent
      (Ondatra zibethica formerly Fiber zibethicus). It
      resembles a rat in color and having a long scaly tail, but
      the tail is compressed, the hind feet are webbed, and the
      ears are concealed in the fur. It has scent glands which
      secrete a substance having a strong odor of musk. Called
      also musquash, musk beaver, ondatra, and sometimes
      water rat.
      [1913 Webster +PJC]

   2. (Zool.) The musk shrew.
      [1913 Webster]

   3. (Zool.) The desman.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Water rat \Wa"ter rat`\
   1. (Zool.)
      (a) The water vole. See under Vole.
      (b) The muskrat.
      (c) The beaver rat. See under Beaver.
          [1913 Webster]

   2. A thief on the water; a pirate.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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