water dog vs water rat

water dog and water rat both are nouns.

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

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

water dog (noun)	water rat (noun)
a dog accustomed to water and usually trained to retrieve waterfowl	any of various amphibious rats
a person who enjoys being in or on the water	common large Eurasian vole
	a person who enjoys being in or on the water