1970's book about a flooded Earth by aliens, floating cities and pirates that patrol the waters

Question

I read a book back probably in the 1970's that had to do with Earth having been flooded by alien off-worlders who then subsequently left...to return at a later date.

The story starts off with a young man who lives on one of the remaining floating cities that pay tribute to 'pirates' for their protection. He decides to join the pirates when they make a port call and through the book works his way up the ranks to eventually be a captain in his own right.

At some point near the end of the book the aliens return and he with the other pirates fight them for possession of Earth.

This was a single book and not a series.

Answer 1

Pretty sure this is Conquerors From The Darkness by Robert Silverberg.

Points that match:

Protagonist is young boy

Dovirr Stargan is a young man who grew up on a city floating on the world sea.

Earth is flooded:

aliens flooded Earth to provide breeding grounds for their young

Pirates trade between cities:

Thalassarchies serve as a cross between pirates and vikings in this world. Wanting to leave his city Dovirr joins them for excitement.

Not a series:

Correct. It is a standalone book. But was part of a broader set of juvenile science fiction called the Winston set. The books all had in common youthful protagonists written by leading science fiction authors of the day.