I lived off water and black coffee...and of course the shakes for the past 4 weeks, and i've just found out that there are some fizzy drinks that you can have. The post is on here somewhere, but just off the top of my memory...you can have some sparking flavoured water, think it was Tesco's and Sainsbury's; Coke Zero; Asda's Diet coke/Zero sugar coke (which i tried yesterday, and is quite good)...there's a few more, but definately try and find it on here, it's a recent post. I think the more variety we have, the less likely we are to 'stray'!
Good luck!!