merchant marine vs cargo ships

merchant marine and cargo ships both are nouns.

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

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

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

Other synonyms of merchant marine include merchant vessels, shipping.

merchant marine (noun)	cargo ships (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