The Collaborative International Dictionary of English v.0.48:

Water dog \Wa"ter dog`\
   1. (Zool.) A dog accustomed to the water, or trained to
      retrieve waterfowl. Retrievers, waters spaniels, and
      Newfoundland dogs are so trained.
      [1913 Webster]

   2. (Zool.) The menobranchus.
      [1913 Webster]

   3. A small floating cloud, supposed to indicate rain.
      [1913 Webster]

   4. A sailor, esp. an old sailor; an old salt. [Colloq.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hellbender \Hell"bend`er\, n. (Zool.)
   A large North American aquatic salamander (Protonopsis
   horrida or Menopoma Alleghaniensis). It is very voracious
   and very tenacious of life. Also called alligator, and
   water dog.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water dog
    n 1: a person who enjoys being in or on the water [syn: water
         dog, water rat]
    2: a dog accustomed to water and usually trained to retrieve
       waterfowl