The Collaborative International Dictionary of English v.0.48:

Dowser \Dows"er\, n.
   1. A divining rod used in searching for water, ore, etc., a
      dowsing rod. [Colloq.]
      [1913 Webster]

   2. One who uses the dowser or divining rod. --Eng. Cyc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dowser
    n 1: someone who uses a divining rod to find underground water
         [syn: water witch, dowser, rhabdomancer]
    2: forked stick that is said to dip down to indicate underground
       water or oil [syn: divining rod, dowser, dowsing rod,
       waterfinder, water finder]