What keeps the people from voting with their feet and leaving? Assuming the monopolist is profit oriented, he leaves the people with enough money to survive after water costs.

What keeps these people from collecting rainwater? What keeps other businesses on the periphery of the region to bring in water cheaply. In your example I assume this would not be possible, maybe the people live on an island or on a different planet?

It seems to me that fixing the price at 199,99 creates incredible incentive for competing businesses to find efficient ways of transporting water to that region.

Lastly, Libertarian theory does not claim that an anarcho-capitalist society would be a utopia where terrible situations do not exist. It merely claims that society would be more just. Is it morally okay to let someone die if you could save their life by giving them half your money? Is it morally okay to force someone to give up half their money in order to save someone's life? Definitely both situations are morally questionable, but the former is perfectly just while the latter is not.