subaquatic vs underwater

subaquatic and underwater both are adjectives.

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

As adjectives, subaquatic and underwater are synonyms defined as:

Other synonyms of subaquatic include subaqueous, submerged, submersed.

subaquatic (adjective)	underwater (adjective)
partially aquatic; living or growing partly on land and partly in water	growing or remaining under water
growing or remaining under water	beneath the surface of the water