water gun vs squirter

water gun and squirter both are nouns.

Word	Noun	Adjective	Verb	Adverb
water gun	Yes	No	No	No
squirter	Yes	No	No	No

As nouns, water gun and squirter are synonyms defined as:

water gun and squirter: plaything consisting of a toy pistol that squirts water

Other synonyms of water gun include squirt gun, water pistol.

water gun (noun)	squirter (noun)
plaything consisting of a toy pistol that squirts water	plaything consisting of a toy pistol that squirts water