There is another aspect to increased prices in a localized disaster scenario. If prices on various goods and materials is significantly elevated, it will incentivize outside providers to bring in whatever is in demand.
So, if water is going for multiples of its retail value, (and not artificially capped by tyrants), then Joe Schmoe can load up his pickup truck at a Sam's Club, and haul a load into the affected area to make a tidy profit on his haul.
This is entrepeneurship at work, the (once) driving force that made this country great.
Price is a signal, and it signifies a greater demand for a good or service. Suppliers will respond more strongly to a higher price point, and will strive to provide that which is in demand.
Thirsty residents get their water (or beans n bullets, etc.), Joe gets a few bucks for his efforts, and everyone is better off than they were before Joe showed up.
Unless, of course, FEMA stops him at a roadblock, confiscates his water, or at least lets him keep it as they turn him around. He's out the gas (kinda like Walmart was after Katrina), and nobody has much reason to send in goods, beyond the natural level of charitable contributions, which while helpful, will never meet the demands of a market like Joe would have.