warship vs war vessel

warship and war vessel both are nouns.

Word	Noun	Adjective	Verb	Adverb
warship	Yes	No	No	No
war vessel	Yes	No	No	No

As nouns, warship and war vessel are synonyms defined as:

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

Other synonyms of warship include combat ship.

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