war vessel vs combat ship

war vessel and combat ship both are nouns.

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

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

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

Other synonyms of war vessel include warship.

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