2

This question already has an answer here:

1970's or 80's book about the future earth.

Aliens invaded, and sunk the land, now there are floating islands, and the aliens come back every so many years to plant eggs in the sea to hatch.

There are mer-people as well as normal humans.

marked as duplicate by Stormblessed, DavidW, Chenmunka, jo1storm, eshier Jun 15 at 18:11

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

2

This is Conquerors from the Darkness by 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.
Goodreads

  • Exactly, thanks, this is the one I was searching for. – Steven Jun 15 at 17:05
  • Hi, Steven! As this is obviously the correct answer (you wrote it to your own question), please accept it with the checkmark to mark this question as answered. Thanks! – Stormblessed Jun 15 at 17:25

Not the answer you're looking for? Browse other questions tagged or ask your own question.