underwater diver vs diver

underwater diver and diver both are nouns.

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

As nouns, underwater diver and diver are synonyms defined as:

Other synonyms of underwater diver include frogman.

underwater diver (noun)	diver (noun)
someone who works underwater	large somewhat primitive fish-eating diving bird of the Northern Hemisphere having webbed feet placed far back; related to the grebes
	someone who dives (into water)
	someone who works underwater