diver vs scuba diver

diver and scuba diver both are nouns.

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

As nouns, scuba diver is a hyponym of diver; that is, scuba diver is a word with a more specific, narrower meaning than diver:

Other hyponyms of diver include deep-sea diver, pearl diver, pearler.

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