You're right, I'm making this assumption. I have to make this assumption assuming there are at least a few possible suppliers (i.e. a few gas stations or convenience stores) because otherwise there would be no supply shock and concomitant price increase.
> and you've got enough bottled water to comfortably supply your community
Here I would say you're begging an important question, which is in regards to who has this and other important information? How would a convenience store owner know that the overall bottled water supply in the area is enough or not - and how could they possible make those calculations in time to properly allocate the water? Prices are a great way to spread information and incentivize people to get creative in finding a solution. If bottled water prices go up 10x, I guarantee you people will find a way to get more bottled water there.
You're also assuming that profit-gouged emergency supplies are necessarily limited. If the local water supply has failed after a hurricane, and you've got enough bottled water to comfortably supply your community, you've still got a captive audience; you can maximize your profits by forcing them to empty their wallets to survive, if you're an asshole.