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.