underwater diver vs explorer

underwater diver and explorer both are nouns.

Word	Noun	Adjective	Verb	Adverb
underwater diver	Yes	No	No	No
explorer	Yes	No	No	No

As nouns, explorer is a hypernym of underwater diver; that is, explorer is a word with a broader meaning than underwater diver:

underwater diver: someone who works underwater
explorer: someone who travels into little known regions (especially for some scientific purpose)

Other hypernyms of underwater diver include adventurer.

underwater diver (noun)	explorer (noun)
someone who works underwater	someone who travels into little known regions (especially for some scientific purpose)