ski vs water ski

ski and water ski both are nouns.

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

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

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

Other hyponyms of ski include roller ski, touring ski.

As verbs, water ski is a hyponym of ski; that is, water ski is a word with a more specific, narrower meaning than ski:

Other hyponyms of ski include wedel, hot-dog, schuss, slalom.

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

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