The Collaborative International Dictionary of English v.0.48:

Water adder \Wa"ter ad"der\ (Zool.)
      (a) The water moccasin.
      (b) The common, harmless American water snake
          (Tropidonotus sipedon). See Illust. under Water
          Snake.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Water moccasin \Wa"ter moc"ca*sin\ (Zool.)
   A venomous North American snake (Ancistrodon piscivorus)
   allied to the rattlesnake but destitute of a rattle. It lives
   in or about pools and ponds, and feeds largely of fishes.
   Called also water snake, water adder, water viper.
   [1913 Webster]