merchant marine vs crew

merchant marine and crew both are nouns.

Word	Noun	Adjective	Verb	Adverb
merchant marine	Yes	No	No	No
crew	Yes	No	Yes	No

As nouns, crew is a hypernym of merchant marine; that is, crew is a word with a broader meaning than merchant marine:

merchant marine (noun)	crew (noun)
conveyance provided by the ships belonging to one country or industry	an organized group of workmen
the crew of a merchant vessel	the men and women who man a vehicle (ship, aircraft, etc.)
	the team of men or women manning a racing shell
	an informal body of friends

merchant marine (verb)	crew (verb)
	serve as a crew member on