It sounds like you see profit as the sole motivation of people's actions.
If someone owned all the water, they would have not just profit-making potential, but they would have a lot of power. In particular, they would have the power of life and death over virtually everyone on the planet.
In a free market utopia, these people could kill as many people as they liked by simply refusing to sell them water, and believers in a completely free market wouldn't lift a finger to stop them -- because, after all they're just freely doing what they like with their own property.
The world is full of people with malicious motives. The prisons are full of them, and there are plenty more outside of prison. Wars, ethnic cleansings, and genocides have killed people by the millions. Some of this was done for profit motives, but some done for other motives.
Don't for a moment think that people like that would hesitate to use the power in their hands to harm those they hated or wanted dead for whatever reasons of their own.
Then there the sociopaths, who would let others die simply because they think giving them water would be worth their bother, or maybe because they were just more interested in other things.
Also, you don't have to buy up all the water in the world to be able to wreak havoc. All the water in a particular water-scarce region might be enough.
Of course, water is just an example and an analogy. The fact is that free market believers have very little to nothing except faith in the free market that would prevent the concentration of wealth and power (in whatever form) and the subsequent abuse of such at the whims of those who wield it.