I do drink a lot of water, and I'm looking for ways to make it more helpful (I have no idea what alkaline means when it comes to drinking, but it's something to look into) but I think the occasional diet soft drink should be fine for now... I intend to quit it completely eventually.
When I was 18, I would buy 2 cartons of 30 cans of Pepsi Max or Coke Zero (I actually preferred the taste, real cola is too strong) and they would be gone within a week. That's how f**king bad I was. I think a 600ml of Max on my weekly shopping trip is an improvement over that.