underwater vs biology

underwater is not a noun while biology is a noun.

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

underwater (noun)	biology (noun)
	the science that studies living organisms
	all the plant and animal life of a particular region
	characteristic life processes and phenomena of living organisms

underwater (adjective)	biology (adjective)
growing or remaining under water
beneath the surface of the water