water rat vs water vole

water rat and water vole both are nouns.

Word	Noun	Adjective	Verb	Adverb
water rat	Yes	No	No	No
water vole	Yes	No	No	No

As nouns, water rat and water vole are synonyms defined as:

Other synonyms of water rat include Arvicola amphibius.

water rat (noun)	water vole (noun)
any of various amphibious rats	common large Eurasian vole
common large Eurasian vole	The largest vole of western North America (Microtus richardsoni)
a person who enjoys being in or on the water