merchant vessels vs merchant marine

merchant vessels and merchant marine both are nouns.

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

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

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

Other synonyms of merchant vessels include cargo ships, shipping.

merchant vessels (noun)	merchant marine (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