I'm not sure what a right to water really means. A lot of rural cities don't even provide water. People have wells and you pay for them yourself.
I think it's covered under that "right to life" section of the Constitution. Deliberately poisoning people through their water supply seems like a sure-fire way to deny the right to life of your citizens.
The US Constitution? Perhaps you are thinking of the Declaration of Independence?
Or the Fifth Amendment, formulating it in the negative: https://en.wikipedia.org/wiki/Fifth_Amendment_to_the_United_...
I believe it is important for us to consider where rights derive from. The Constitution and Bill of Rights are designed to protect rights that humans already have ("endowed by their Creator") and prevent the government from taking them away, or infringing on them. The law can grant ordinary civil rights, above and beyond intrinsic human rights, but if the Constitution isn't protecting the latter, then there is a problem.