You have to steer clear of anything that contains citric acid and anything that may contain ingredients of flowers or berries.
Even the flavoured drinks that do not contain these are proven in some people to trigger cravings that can cause you to break the diet.
What I have done in the past, and still do, is use the CD water flavourings (work out how much you need in a pint glass to get the strength you want), and then add sparkiling mineral water to it ...... in other words I make my own completely legal fizzy drink.
May not be coke, but it hits the spot believe me!
Steve