crewman vs water rat

crewman and water rat both are nouns.

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

As nouns, water rat is a hyponym of crewman; that is, water rat is a word with a more specific, narrower meaning than crewman:

Other hyponyms of crewman include hand, lascar, gob, Jack-tar, Jack, mariner, old salt, sea dog, seafarer, seaman, tar, water dog, yachtsperson.

crewman (noun)	water rat (noun)
a member of a work crew	any of various amphibious rats
a member of a flight crew	common large Eurasian vole
any member of a ship's crew	a person who enjoys being in or on the water