merchant marine vs merchant vessels

merchant marine and merchant vessels both are nouns.

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

As nouns, merchant marine and merchant vessels are synonyms defined as:

merchant marine and merchant vessels: conveyance provided by the ships belonging to one country or industry

Other synonyms of merchant marine include cargo ships, shipping.

merchant marine (noun)	merchant vessels (noun)
conveyance provided by the ships belonging to one country or industry	conveyance provided by the ships belonging to one country or industry
the crew of a merchant vessel