water rat vs gnawer

water rat and gnawer both are nouns.

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

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

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

Other hypernyms of water rat include rodent.

water rat (noun)	gnawer (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