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 snake \Wa"ter snake`\ (Zool.)
      (a) A common North American colubrine snake (Tropidonotus
          sipedon) which lives chiefly in the water.
      (b) Any species of snakes of the family Homalopsidae,
          all of which are aquatic in their habits.
          [1913 Webster]