Please note that "just" and "fair" are not synonyms. What's fair to you may not be fair to me. In the words of Bastiat: justice is the lack og injustice. Some action or inaction is just if no one's rights were violated. If you violate someone's property rights to safe a life, this is not justice but injustice. Undertake this action at your own peril.

To be clear, a right to water can certainly not exist, as it would be in conflict with another's right to property. It is important that a system of rights contain no contradictions, and therefore a law can only rise to the status of right if it is universal. Similarly a right to healthcare entitles you to a docto's labour which violates his rights.

Let it be clear that I find your sketched situation a very bad one and I pity the people, but also the situation is highly unrealistic

It is much more profitable for the monopolist to leave the area alive. In that way he does not need to move.

If the monopolist buys all this water his profits dwindle, and the market mechanism does its job. The demand for water is still not satisfied and more capitalists can come in and do the same.