An all out thermonuclear war would be the end of the human race. Thankfully, it won't happen simply because thermonuclear warfare is now obsolete. An all-out electromagnetic nuclear war is its successor, which would only spell the end of our civilization. But chances are we just devolve into civil wars breaking out and a few viruses. Our base case is a 25-33% fall in population in one generation. Bottled water won't help. Large rain water collection systems are a lot more likely to give a survival chance.