fish duck vs sea duck

fish duck and sea duck both are nouns.

Word	Noun	Adjective	Verb	Adverb
fish duck	Yes	No	No	No
sea duck	Yes	No	No	No

As nouns, sea duck is a hypernym of fish duck; that is, sea duck is a word with a broader meaning than fish duck:

fish duck: large crested fish-eating diving duck having a slender hooked bill with serrated edges
sea duck: any of various large diving ducks found along the seacoast: eider, scoter, merganser

fish duck (noun)	sea duck (noun)
large crested fish-eating diving duck having a slender hooked bill with serrated edges	any of various large diving ducks found along the seacoast: eider, scoter, merganser