The only thing I think you need to be aware of is that carbonated water can make you bloated if you have too much of it. And of course it's got to be unflavoured.
Fizzy water is my 'going to the pub and not drinking drink'! I am planning on drinking about 6 pints of the stuff tonight. It has never affected my weightloss (but then I lose weight veeerrry slowly) but it can make me burp
I only drink carbonated as get board of still water all the time! I have about 4 litres of carbonated a day and it hasn't done me any harm and to be fair I also drink Sainsbury's unsugared flavoured waters as well and they have never affected me!