Even with a question mark, 'please can you drink the water' is not a question but a polite imperative. Without a question mark, there is no ambiguity at all. This is a commonplace English usage and should be allowed.
Couldn't this sentence also mean what someone would think (or quietly whisper) while hoping that it happens? For example, if you're doing a prank (by adding something to the water), and the person seems like they won't fall for it, you would then think/this, no? Or the phrase in Italian would be different?