water system vs water supply

water system and water supply both are nouns.

Word	Noun	Adjective	Verb	Adverb
water system	Yes	No	No	No
water supply	Yes	No	No	No

As nouns, water system and water supply are synonyms defined as:

water system and water supply: a facility that provides a source of water

Other synonyms of water system include water.

water system (noun)	water supply (noun)
a facility that provides a source of water	a facility that provides a source of water
a river and all of its tributaries