"He is not drinking water."
Translation:On nie pije wody.
Although the personal pronouns are so often omitted, it is not that common with 3rd person pronouns. After all, the pronoun here could be he/she/it, so it's rather needed, unless it was made obvious in a previous sentence. But there is no previous sentence here, so the pronoun should stay.
Although having written that, I checked and "Nie pije wody" is actually accepted despite that. Well, okay.