Back in the '80s, I read a story - novel length for the time (would probably be a 'novella' now) - with the following aspects that I can recall:
The world had been 'drowned'; except for outcroppings from the highest mountains, there was no natural land - only 'cities' anchored in place somehow.
The world was divided into a number of "Thassalarchies", sections of ocean ruled by a strongman who commanded a fleet.
Technology seemed to have regressed to more-or-less early Renaissance
There was a race of modified humans, the 'Seaborn Ones', who lived under the seas.
The viewpoint character was a peon of the time, and wanted to go to sea and become one of the Thassalarch's men.
At the end, the viewpoint character had managed to ascend to being the Thassalarch of two territories, and had learned how to communicate with the Seaborn Ones, more-or-less just in time for the "return of the Star Beasts", who supposedly caused the drowning of Earth. In the end, the Star Beasts were defeated.
I will be d$$$ed if I can recall the title or author of the story. Help, please?