The Collaborative International Dictionary of English v.0.48:

Sea duck \Sea" duck`\ (Zool.)
   Any one of numerous species of ducks which frequent the
   seacoasts and feed mainly on fishes and mollusks. The
   scoters, eiders, old squaw, and ruddy duck are examples. They
   may be distinguished by the lobate hind toe.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sea duck
    n 1: any of various large diving ducks found along the seacoast:
         eider; scoter; merganser