What consequences? Dying of thirst is a heavier burden than unburdening the greedy water hoarder's excess.
Getting shot in self-defense, for instance.
The libertarian position is that rights are always valid at all times. There is no such thing as "oh no we declare a state of emergency your property rights are now invalid sorry"
There are so many things that can be done in your hypothetical. Taking a loan. Selling possessions to scrounge up the money.
Or simply driving away to another place to get water.
Or posting on social media that there is a water shortage and get people from outside to come to you with water.
If your first thought is to use violence to expropriate somebody who has done nothing wrong, then that says hell of a lot about you. I don't take kindly to your kind.
Other than that, your hypothetical is a lot like Vaush's coconut tree analogy.
Which, again, is not something to hypothesize a free market, but rather a hypothetical about the dangers of monopolies.
I recommend you watch this video:
Let me know if you disagree with anything in the video.