water tube vs tubing

water tube and tubing both are nouns.

Word	Noun	Adjective	Verb	Adverb
water tube	Yes	No	No	No
tubing	Yes	No	No	No

As nouns, tubing is a hypernym of water tube; that is, tubing is a word with a broader meaning than water tube:

water tube: a boiler tube through which the water flows.
tubing: conduit consisting of a long hollow object (usually cylindrical) used to hold and conduct objects or liquids or gases

Other hypernyms of water tube include tube.

water tube (noun)	tubing (noun)
a boiler tube through which the water flows.	conduit consisting of a long hollow object (usually cylindrical) used to hold and conduct objects or liquids or gases