Relating to, occurring, used, or performed beneath the surface of water.(adjective)
Nautical Below the water line of a vessel.(adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "underwater" in a sentence
"Sora, one of the boys who has the ability to survive underwater, is missing from the hospital."
"But, when you add in underwater work, heavy CGI work and wire work, it becomes extraordinarily complicated."
"It was a tall round shape made of smooth glass, and it was tied with a fishing line, Orphan discovered, to a rusting hook that protruded underwater from the side of the embankment."