Sea-Slums
(AKA: The ruins of the 2nd floating city, Atlantis)
Not the biggest city, but probably the strangest. Although all of the floating cities sank after the massive power failure that heralded the end of modern society, they did not all sink evenly. Due to it's closeness to the coast, much of the Atlantis city remained somewhat livable and right-side up.
Massive nets are strung along the borders to keep flotsam and dangerous aquatic things out, with varying levels of success. The water is high in various contaminants as well, not something anyone wants to swim in unless it's a life or death situation. Little lives in the polluted water except the most adaptable of lifeforms. Attempts to introduce fish kimera are largely ineffective, but well-regulated attempts continue to be made.
Water is the most expensive resource, and people collect it every way they can, condensation in particular.
Technology: Electricity comes from mini hydroelectric dams and private windmills on the tops of the taller skyscrapers. Although it's use is heavily monitored, it's fairly reliable and rarely does the main part of the city fall into blackouts. Rarely, in this case, means only about once or twice a week. Almost no factories exist since it's far too difficult to get raw materials in the world's current state.
Society: The tallest livable building belong to the wealthiest people. Nobles would be a good description of them. Law-makers, judges, and the king-like mayor, they often gain their influence by owning lakes since fresh water is such a much-needed resource throughout the city.
Below them are the middle class, they typically live in somewhat crowded apartments and have a row boat or two as well as some 'modern' luxuries like TV, radio, refrigerators, and videogames. They get by through raising domestic animals like chickens and goats(cows are extinct in the city), growing crops on the apartment roofs and docks, making goods like textiles, or anything else they can think of. Aquatic crops are being developed, but most have yet to be particularly useful. No one fishes for food if they can help it because what few fish there are contain various chemicals like pesticides and mercury.
The poor class lives outside the city limits, has little or no electricity, and typically doesn't live long for various reasons. Carnivorous mutant fish and wild kimera are common reasons.