How would you say "Do not drink the water"? Or is that also an acceptable translation?
Hindi doesn't have a word for 'the' but it has an equivalent of 'that'. So if we wanted to get specific, instead of saying 'don't drink the water' in Hindi, we would say 'don't drink that water' which would basically mean the same.
Don't drink that water = उस पानी को मत पीओ।
You added another को particle as well that was not in the original sentence. Is that also necessary? And why उस for "that" rather than वह?
उस for "that" rather than वह because of the postposition को but I don't know why को was used in the first place.
If you saw a sign next to a river saying "Don't drink the water," would it even have उस in it? Or would it be enough to just say "पानी मत पियो"?
Because if you don't need any indication of "the" in a normal context, then "Don't drink the water" should be accepted as a translation for this. (Posting this comment because I also tried it and was rejected.)