Seconded. It’s really very good. Cameron liked it so much that during filming he gave the three leads - Ed Harris, Mary Elizabeth Mastrantonio and Michael Biehn - the backstory chapters on their characters so they’d have a better feel for who they were.
Michael Crichton’s Sphere takes place mostly underwater (it starts on the surface, at least). As for recommending it…well, it’s better than the movie (not saying much), but it really falls apart towards the end, with some chunks of noticeable rot on the way there. It’s mostly a fun ride while it lasts, though.
“Crisis on Conshelf Ten.” YA Sci-fi by Monica Hughes.
Doesn’t entirely take place underwater, but the majority of the action is down there. The son of the Moon president, (also the first child born and raised on the moon,) has problems with Earth-normal gravity tagging along with his father on a diplomatic trip to the UN, so he goes to stay with cousins in an undersea settlement, where he ends up mixed into the middle of a mystery and a revolution.