combat ship vs war vessel

combat ship and war vessel both are nouns.

Word	Noun	Adjective	Verb	Adverb
combat ship	Yes	No	No	No
war vessel	Yes	No	No	No

As nouns, combat ship and war vessel are synonyms defined as:

combat ship and war vessel: a government ship that is available for waging war

Other synonyms of combat ship include warship.

combat ship (noun)	war vessel (noun)
a government ship that is available for waging war	a government ship that is available for waging war