The Collaborative International Dictionary of English v.0.48:

Water shrew \Wa"ter shrew`\ (Zool.)
   Any one of several species of shrews having fringed feet and
   capable of swimming actively. The two common European species
   (Crossopus fodiens, and Crossopus ciliatus) are the best
   known. The most common American water shrew, or marsh shrew
   (Neosorex palustris), is rarely seen, owing to its
   nocturnal habits.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water shrew
    n 1: any of several small semiaquatic shrews usually living near
         swift-flowing streams