swim vs skin-dive

swim and skin-dive both are nouns.

Word	Noun	Adjective	Verb	Adverb
swim	Yes	No	Yes	No
skin-dive	Yes	No	Yes	No

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

swim: the act of swimming
skin-dive: underwater swimming without any more breathing equipment than a snorkel

Other hyponyms of swim include bathe, skinny-dip, dip, plunge, dive, diving, floating, natation, skin diving.

swim (noun)	skin-dive (noun)
the act of swimming	underwater swimming without any more breathing equipment than a snorkel

swim (verb)	skin-dive (verb)
travel through water	swim underwater with no breathing apparatus other than a snorkel
be afloat either on or below a liquid surface and not sink to the bottom
move as if gliding through water
be covered with or submerged in a liquid
be dizzy or giddy