water gate vs floodgate

water gate and floodgate both are nouns.

Word	Noun	Adjective	Verb	Adverb
water gate	Yes	No	No	No
floodgate	Yes	No	No	No

As nouns, water gate and floodgate are synonyms defined as:

water gate and floodgate: regulator consisting of a valve or gate that controls the rate of water flow through a sluice

Other synonyms of water gate include head gate, penstock, sluice valve, sluicegate.

water gate (noun)	floodgate (noun)
regulator consisting of a valve or gate that controls the rate of water flow through a sluice	regulator consisting of a valve or gate that controls the rate of water flow through a sluice
	something that restrains a flood or outpouring