first mate vs ship's officer

first mate and ship's officer both are nouns.

Word	Noun	Adjective	Verb	Adverb
first mate	Yes	No	No	No
ship's officer	Yes	No	No	No

As nouns, ship's officer is a hypernym of first mate; that is, ship's officer is a word with a broader meaning than first mate:

first mate: the officer below the master on a commercial ship
ship's officer: a person authorized to serve in a position of authority on a vessel

Other hypernyms of first mate include officer.

first mate (noun)	ship's officer (noun)
the officer below the master on a commercial ship	a person authorized to serve in a position of authority on a vessel