tanker vs water truck

tanker and water truck both are nouns.

Word	Noun	Adjective	Verb	Adverb
tanker	Yes	No	No	No
water truck	Yes	No	No	No

As nouns, water truck is a hyponym of tanker; that is, water truck is a word with a more specific, narrower meaning than tanker:

tanker: a motor vehicle designed to carry liquefied loads, dry bulk cargo or gases on roads.
water truck: a tank truck used to haul and disperse water.

Other hyponyms of tanker include water tank truck.

tanker (noun)	water truck (noun)
a cargo ship designed to carry crude oil in bulk	a tank truck used to haul and disperse water.
a motor vehicle designed to carry liquefied loads, dry bulk cargo or gases on roads.
a soldier who drives a tank