snorkel diving vs skin diving

snorkel diving and skin diving both are nouns.

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

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

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

Other hypernyms of snorkel diving include skin-dive.

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