The only time water will be a problem is when someone's using weapons to force others away from sources of it. In terms of desalinating water and delivering it to thirsty people, that's something the market is perfectly capable of attaining. It's always profitable to meet unmet basic needs so any water holes will be filled automatically. And that's in the worst case scenario, where people are just buying trucked in water. In most places there will be actual plumbing serving people.