A better example would be the Phoenicians, who were a thalassocratic civilisation based around all edges of the Mediterranean Sea (North Africa, the Middle East

A better example would be the Phoenicians, who were a thalassocratic civilisation based around all edges of the Mediterranean Sea (North Africa, the Middle East and Southern Europe) who (IIRC, not a historian) were predominantly traders. If I were to guess at how seasteaders would make money? Bringing things to you that you'd normally have to travel for (or are illegal) and pay a lot of money - medical and cosmetic tourism, gambling, that sort of thing.