The Collaborative International Dictionary of English v.0.48:

Waterfowl \Wa"ter*fowl`\, n.
   Any bird that frequents the water, or lives about rivers,
   lakes, etc., or on or near the sea; an aquatic fowl; -- used
   also collectively.
   [1913 Webster]

   Note: Of aquatic fowls, some are waders, or furnished with
         long legs; others are swimmers, or furnished with
         webbed feet.
         [1913 Webster]

WordNet (r) 3.0 (2006):

waterfowl
    n 1: freshwater aquatic bird [syn: waterfowl, water bird,
         waterbird]