underwater diver vs adventurer

underwater diver and adventurer both are nouns.

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

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

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

Other hypernyms of underwater diver include explorer.

underwater diver (noun)	adventurer (noun)
someone who works underwater	a person who enjoys taking risks
	someone who travels into little known regions (especially for some scientific purpose)