swimmer vs skin-diver

swimmer and skin-diver both are nouns.

Word	Noun	Adjective	Verb	Adverb
swimmer	Yes	No	No	No
skin-diver	Yes	No	No	No

As nouns, skin-diver is a hyponym of swimmer; that is, skin-diver is a word with a more specific, narrower meaning than swimmer:

swimmer: a person who travels through the water by swimming
skin-diver: an underwater swimmer equipped with a face mask and foot fins and either a snorkel or an air cylinder

Other hyponyms of swimmer include floater, aquanaut, surfboarder, surfer.

swimmer (noun)	skin-diver (noun)
a trained athlete who participates in swimming meets	an underwater swimmer equipped with a face mask and foot fins and either a snorkel or an air cylinder
a person who travels through the water by swimming