sea captain vs ship's officer

sea captain and ship's officer both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea captain	Yes	No	No	No
ship's officer	Yes	No	No	No

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

sea captain: an officer who is licensed to command a merchant ship
ship's officer: a person authorized to serve in a position of authority on a vessel

Other hypernyms of sea captain include officer.

sea captain (noun)	ship's officer (noun)
an officer who is licensed to command a merchant ship	a person authorized to serve in a position of authority on a vessel