water heater vs tank

water heater and tank both are nouns.

Word	Noun	Adjective	Verb	Adverb
water heater	Yes	No	No	No
tank	Yes	No	Yes	No

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

Other hypernyms of water heater include heater, warmer, storage tank.

water heater (noun)	tank (noun)
a heater and storage tank to supply heated water	a large (usually metallic) vessel for holding gases or liquids
	an enclosed armored military vehicle; has a cannon and moves on caterpillar treads
	a freight car that transports liquids or gases in bulk
	a cell for violent prisoners
	as much as a tank will hold

water heater (verb)	tank (verb)
	treat in a tank
	consume excessive amounts of alcohol
	store in a tank by causing (something) to flow into it