~70% of the ocean is covered in water, while humanity fights for the tiny scraps of land. But what if there was a way around it? Just build some more, float a city on the water, use the considerable force of the ocean for power, use modern efficient farming and fishing techniques for food, zippy integrated transport as a key design feature rather than something of an afterthought.
But there's always going to be trouble in the big city. A floating, oceangoing city will have its own unique troubles to deal with. It's been touched on a few times, often in games and movies. Everything from Waterworld to the recent(ish) game Brink, feature big floating cities with a whole mess of trouble in them.
But what if it mostly worked? Despite having to button everything up for sea storms, the dangers of pirates and hostile foreign parties who might not consider the floating city to be part of a nation and therefore up for grabs, seedy businesses looking for places to do their thing outside of normal country's laws, it was more or less a reasonable place to live? Sounds like a good spot for some Player Characters to find work, stir up trouble, or both.