Conquerors from the Darkness — Robert Silverberg

A thousand years in the future, the earth has been conquered by an alien race and covered by a single sea. Dovirr Stargan, who is disgusted with the servility of his life on the floating city of Vythain, longs to become one of the Sea-Lords, who roam the sea as powerful protectors of the cities. Dovirr gets his wish, but the return of the alien race brings unexpected and critically dangerous crises to his new life as he learns the real, sometimes terrible, significance of power.