If there is a fixed supply of water, what better mechanism than price exists to efficiently determine the best use of that water? Not allowing people to raise prices means you end up with wildly inefficient allocation of resources (i.e. first guy in line says, "Man, there's a remote chance this flooding lasts for 2 weeks. Water is the same price it was pre-hurricane? I'll take all you've got.")
Similarly, not allowing people to raise prices actually decreases supply. If you remember post-Katrina, there were numerous "anti-price gouging" orders enacted. The result? All the people who were willing to drive a truck full of generators (and plywood, and water, etc. etc.) 1000 miles to make a few bucks/hundred bucks profit per unit now had no incentive to do so. And so they didn't. And the people of New Orleans suffered needlessly.