Hi Mikkel, You can say: turn on the water, turn on the faucet or turn on the tap. All are correct. You can also say, turn on the hot/cold water. Open the water, is not correct. If there is a valve on a pipe, you could say, open the water valve. You would still say, turn on the water. Hope this helps!

Nope. Ain't natural at all. We can turn on the tap/faucet, and turn on the water. But the concept of opening water ( or gas, or anything else that flows) has no logic for the English-speaking mind. We open doors and windows to let in air or light. We can open a valve to allow the water or gas to flow. On a canal, for example, you might open the lock gates to let the water in or out. But "open the water"? No, it makes no sense. Another giveaway for the undercover spy, I'm afraid.

'turn on the tap' is the most natural way to say it. alternatively there is 'run the water' as well.