Jokes apart, 'the water is drunk' is surely a correct English translation? I asked a Greek friend of mine who agreed. He also said that 'the water is drinkable' would be 'το νερό είναι πόσιμο'. Sorry to add to the clamour... ;-)
"The water is drunk." has been added...however in my view it is a very odd Eng. sentence and I'd never use it nor allow my students to use it. In this case "Το νερό πίνεται." and "Το νερό είναι πόσιμο'." are interchangeable and both are accepted as translations.
so, what could be a right answer in english to express the fact that someone is drinking water ? maybe only with the active voice ?
I think, yes, this would only be expressed using the active voice. "Is being drunk" would express the correct meaning but sounds rather odd. "Is drunk" could mean "is all drunk up," but anyone who hears it will interpret it as "is intoxicated."
Why is the water is drunk wrong? the answer given is, as you say, the water's drunk which means exactly the same!
Το νερό πίνεται translates as the water is drinkable but how do you say A drinkable water? Or edible fruit as opposed to The fruit is edible? And so on with some other words such as watchable, enjoyable, doable, etc. Thank you.
There are adjectives for that, το νερό πίνεται is just another way to say it. drinkable=πόσιμος, edible=φαγώσιμος, enjoyable=απολαυστικός, doable=εφικτός etc.