The 13th Continuum (Continuum trilogy, #1)

One thousand years after a cataclysmic event leaves humanity on the brink of extinction, the survivors take refuge in continuums designed to sustain the human race until repopulation of Earth becomes possible. Against this backdrop, a group of young friends in the underwater Thirteenth Continuum dream about life outside their totalitarian existence, an idea that has been outlawed for centuries. When a shocking discovery turns the dream into a reality, they must decide if they will risk their own extinction to experience something no one has for generations, the Surface.