Most of them I've seen, say 0 calories but there is an artificial sweeteners.
As far as truely no sweetener, carbonated water... I don't think enough studies have been done...
With some claiming the carbonation can hurt calcium in your bones and or your teeth, but others have claimed that's wrong or extremely extremely minimum.
Overall, I haven't found proof of anything too negative with carbonated water that contains no sweeter.
Seems like the sweetest are clearly the bigger potential issue.