Simple: The real world is not made up out of things which are independent, but everything is connected one way or another. If a private company has control over water they want to grow and maximize profits. First consequence is clearly that prices will go up, even more so if there is clear demand for that, and secondly they will slurp as much as they can out of the ground. Unfortunately, the water in the ground is not infinite, it is a shared resource and needs time to get replenished. It is part of this "common good". So you have to start regulating, and also controlling the regulation of how much at most a company can draw water out of the ground. Same goes for polluting water (not just by dumping dirty water, but think fracking and other such things) etc.

Same goes for land use. Agriculture is a main driving factor of mass extinction (yes, we are in the midst of a biosphere mass extinction event) because they ignore the common good, and other land uses bring their share of problems with it.