The Collaborative International Dictionary of English v.0.48:

Water witch \Wa"ter witch`\ (Zool.)
   (a) The dabchick.
   (b) The stormy petrel. [Prov. Eng.]
       [1913 Webster]

WordNet (r) 3.0 (2006):

water witch
    n 1: someone who uses a divining rod to find underground water
         [syn: water witch, dowser, rhabdomancer]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "water witch":
   Comus, Faust, conjurer, diabolist, diviner, divining rod,
   doodlebug, dowser, dowsing, dowsing rod, miracle-worker,
   necromancer, sorcerer, thaumaturge, thaumaturgist, theurgist, wand,
   warlock, water witching, witching, witching stick, wizard