water ski vs ski

water ski and ski both are nouns.

Word	Noun	Adjective	Verb	Adverb
water ski	Yes	No	Yes	No
ski	Yes	No	Yes	No

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

water ski: broad ski for skimming over water towed by a speedboat
ski: narrow wood or metal or plastic runners used in pairs for gliding over snow

As verbs, ski is a hypernym of water ski; that is, ski is a word with a broader meaning than water ski:

water ski (noun)	ski (noun)
broad ski for skimming over water towed by a speedboat	narrow wood or metal or plastic runners used in pairs for gliding over snow

water ski (verb)	ski (verb)
ride water skis	move along on skis