There's an old medical saying, often attributed to the followers of the alchemist Paracelsus, that the dose makes the poison. Paracelsus had argued that there was no dividing line between poisonous and non-poisonous substances; every substance was a poison, and the what made the difference between a remedy and a poison was simply how much you took. Chemicals in a large enough dose can kill you. If you ate twenty pounds of spinach in one sitting, for instance, there is a good chance that you would die of oxalic acid poisoning. And you can poison yourself with water.
As you might expect, dying through being poisoned by water is a very rare phenomenon. In order to suffer water intoxication, you have to drink enough water in a short enough period of time to reduce your electrolytes to such a low level that bodily functions begin shutting down. People have died from water intoxication in water-drinking contests, under conditions of intense exercise (marathon runners have to be careful not to rehydrate too quickly), or under conditions of extreme heat. So now you know: hydrogen hydroxide is a poisonous substance that in sufficiently large amounts can disrupt the electrolyte balance of the body, leading to kidney failure and even death. And it is everywhere. In reality, of course 'sufficiently large amounts' are very large amounts indeed.
The dose-makes-the-poison maxim occasionally gets criticized in toxicology, by the way. Here are two examples:
Does 'the dose make the poison'?
The Dose Makes the Poison -- or Does It?
(The second one is especially good.) In reality, these criticisms seem to me mostly to be a symptom of a common problem in fields of research: claims are first associated with a very particular operational approach to discussing or applying them and then, over time, simply identified with them because it is simpler. This, however, often involves confusing a true general claim with a particular implication of the claim in a particular context, with the result that the latter is falsely generalized. Appropriate dose ranges for a chemical, like the Aristotelian mean for a passion, vary according to the circumstances, although in coherent ways; in practice one can easily lose sight of this, however, and switch into mechanical application of tests and rules.