crew vs merchant marine

crew and merchant marine both are nouns.

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

As nouns, merchant marine is a hyponym of crew; that is, merchant marine is a word with a more specific, narrower meaning than crew:

Other hyponyms of crew include air crew, aircrew.

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

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