water heater vs hot-water heater

water heater and hot-water heater both are nouns.

Word	Noun	Adjective	Verb	Adverb
water heater	Yes	No	No	No
hot-water heater	Yes	No	No	No

As nouns, water heater and hot-water heater are synonyms defined as:

water heater and hot-water heater: a heater and storage tank to supply heated water

Other synonyms of water heater include hot-water tank.

water heater (noun)	hot-water heater (noun)
a heater and storage tank to supply heated water	a heater and storage tank to supply heated water