snorkeling vs skin diving

snorkeling and skin diving both are nouns.

Word	Noun	Adjective	Verb	Adverb
snorkeling	Yes	No	No	No
skin diving	Yes	No	No	No

As nouns, skin diving is a hypernym of snorkeling; that is, skin diving is a word with a broader meaning than snorkeling:

snorkeling: skin diving with a snorkel
skin diving: underwater swimming without any more breathing equipment than a snorkel

Other hypernyms of snorkeling include skin-dive.

snorkeling (noun)	skin diving (noun)
skin diving with a snorkel	underwater swimming without any more breathing equipment than a snorkel