water company vs public utility

water company and public utility both are nouns.

Word	Noun	Adjective	Verb	Adverb
water company	Yes	No	No	No
public utility	Yes	No	No	No

As nouns, public utility is a hypernym of water company; that is, public utility is a word with a broader meaning than water company:

water company: a public utility that provides water
public utility: a company that performs a public service; subject to government regulation

Other hypernyms of water company include public-service corporation, public utility company, utility.

water company (noun)	public utility (noun)
a public utility that provides water	a company that performs a public service; subject to government regulation