sea duck

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various diving ducks of coastal areas, such as an eider or scoter.

from The Century Dictionary.

• noun A duck of the family Anatidæ and subfamily Fuligulinæ, having the hind toe lobate, and often found on salt water. (See Fuligulinæ.)
• noun Specifically, the eider-duck.

from the GNU version of the Collaborative International Dictionary of English.

• (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Alternative spelling of seaduck.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of various large diving ducks found along the seacoast: eider; scoter; merganser