Recs if you like A Darkling Sea...

Life began underwater. Humans yearn to go back, and sometimes, they really shouldn't... A crowd-sourced list of stories that take place primarily underwater.

ocean planet colonisation and deep-sea extraction technologies

At Nohara66's request, scfi books "without space and physics!" (Or as close as can be). Some of these do feature spaceships in the background or as a step from point A to B, or set within galactic empires, or once colonised worlds, but involve no physics and don't focus on life/travel in space.