water rat vs rodent

water rat and rodent both are nouns.

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

As nouns, rodent is a hypernym of water rat; that is, rodent is a word with a broader meaning than water rat:

water rat: any of various amphibious rats
rodent: relatively small placental mammals having a single pair of constantly growing incisor teeth specialized for gnawing

Other hypernyms of water rat include gnawer.

water rat (noun)	rodent (noun)
any of various amphibious rats	relatively small placental mammals having a single pair of constantly growing incisor teeth specialized for gnawing
common large Eurasian vole
a person who enjoys being in or on the water