water boy vs waterer

water boy and waterer both are nouns.

Word	Noun	Adjective	Verb	Adverb
water boy	Yes	No	No	No
waterer	Yes	No	No	No

As nouns, water boy and waterer are synonyms defined as:

water boy (noun)	waterer (noun)
an assistant who supplies drinking water	an assistant who supplies drinking water
	someone who waters plants or crops