subaqueous vs underwater

subaqueous and underwater both are adjectives.

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

As adjectives, subaqueous and underwater are synonyms defined as:

Other synonyms of subaqueous include subaquatic, submerged, submersed.

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