Flood

Interesting story - imagine some source of large amounts of water buried under the crust which, fo reasons no one understands, suddenly bursts through to the surface and floods the Earth under about a kilometer of water. People get together, countries break down, oligarchs build special ships, and other people build rafts.
Some parts were too fantastic to be believable. In London, they only worry about sewage pollution when Greenwich flood barrier is overwhelmed, whereas we in the real UK suffer from sewage pollution after light rain due to lack of investment. I also thought the Middle East would be nuked long before it happens in the book, as each religion fights over Jerusalem.