sea duck

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of various diving ducks, such as the eider or scoter, of coastal areas.

Century Dictionary and Cyclopedia

1. n. A duck of the family Anatidæ and subfamily Fuligulinæ, having the hind toe lobate, and often found on salt water. (See Fuligulinæ.) There are many species, to only one of which the name pertains without a qualifying word. (See def. 2.) The antithesis is river-duck; but many sea-ducks—that is, Fuligulinæ— are found inland. See cuts under Nyroca, Œdemia, eider, canvasback, redhead, pied, scaup, scoter, and surf-duck.
2. n. Specifically, the eider-duck.

Wiktionary

1. n. alternative spelling of seaduck.

GNU Webster's 1913

1. (Zoöl.) 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.

WordNet 3.0

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