Some say Coke Zero is OK as a treat as it has malic acid and not citric so it won't knock you out of ketosis. I personally have one or 2 cans a week as a treat. But I don't think you should count it toward your water intake.
It helped me a great deal at the start. I also did some ice lolly with the water flavour and it helped too.
I hate drinking but found a way around it: I use a soda machine so I get fizzy water and then add some CD flavour. Delicious