H2O (re-released as The Rain), by Virginia Bergin
It’s hard to find a novel populated with young folks that deals realistically with the gritty, awful truths of a world-ending disaster. This one does. It’s not for the faint of heart, and it’s not one of those stories where out plucky heroine somehow rises above and conquers the world. But it’s a good story, absorbing, sometimes disturbing, sometimes sad, but always worth the journey.