cargo ships vs merchant marine

cargo ships and merchant marine both are nouns.

Word	Noun	Adjective	Verb	Adverb
cargo ships	Yes	No	No	No
merchant marine	Yes	No	No	No

As nouns, cargo ships and merchant marine are synonyms defined as:

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

Other synonyms of cargo ships include merchant vessels, shipping.

cargo ships (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