As a native speaker, I wonder: is the problem with the water, or with the bottle?
That said, a few expressions come to mind. The first one is the most direct:
You might not want to drink from that bottle; Ann already had her mouth on it.
The second is more of a euphemism than a scientific fact:
Don't drink from that bottle – it has germs.
(We don't know for a fact that the water has any contaminants, but the word germs is often used to refer to unseen microorganisms that could spread disease, particularly in informal speech.)
Because the concern is mainly with someone else drinking from the bottle, you could also say:
Be careful! That may have someone's backwash in it.
TFD labels this definition of backwash as "informal", while the Urban Dictionary says:
Backwash is often created inadvertently or unintentionally when liquid escapes the mouth during the process of drinking .. When multiple people drink from the same container, there will usually be some amount of backwash put back into the container.
As a footnote, the adjective for drinkable water is potable, though anyone who would deem bottled water as non-potable simply because someone else drank from the bottle is probably using extreme hyperbole, or else is an overly sensative germaphobe.