Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. (Zo["o]l.) The menobranchus.

   3. A small floating cloud, supposed to indicate rain.

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

Source : WordNet®

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