Cities at Sea

Climate change has destroyed land-based civilization. Forewarned, all major coastal cities saved themselves from destruction by building gigantic and highly sophisticated rafts, self supporting, self governing and and totally independent. Life in the cities at sea is very comfortable although strictly disciplined and controlled. Among other things, natural gestation and birth are not permitted, fertilisation is done in the laboratory, embryos are incubated and decanted in their time.
Sal, a young trainee navigator in Sydney, finds things dull and seeks adventure. Inspired by the ancient legend of the little mermaid Rusalka, she dreams of living in the sea with the fishes. She learns of a weird but brilliant geneticist, Jezzy, with a laboratory on the Shanghai raft. Jezzy can modify young humans by giving them gills. Sal moves to Shanghai to join this eugenic project and undergoes the necessary operation. There is fierce opposition to from the orthodox rafters. This compels the modified humans to take drastic action in self defense. They are brought into contact with some surviving land dwellers.