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]

The Collaborative International Dictionary of English v.0.48:

cottonmouth \cottonmouth\ n.
   venomous semiaquatic snake (Agkistrodon piscivorus) of
   swamps in southern U.S.; -- called also water moccasin,
   cottonmouth moccasin, and cottonmouth water moccasin. It
   grows to a length of about 4 feet. The name refers to the
   whiteness of the lips and inside of the mouth.

   Syn: water moccasin, cottonmouth moccasin, Agkistrodon
        piscivorus.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

water moccasin
    n 1: venomous semiaquatic snake of swamps in southern United
         States [syn: water moccasin, cottonmouth, cottonmouth
         moccasin, Agkistrodon piscivorus]
    2: any of numerous North American water snakes inhabiting fresh
       waters