Hiya, yep we're allowed Sparkling water and soda water but NOT tonic water (it's got sugar in it).
Soda water does have a small amount of salt in it (as the bicarbonate) but it's actually less than most Mineral waters have in them. (or so i'm told anyway! lol)
I must admit that i love having sparkling as it makes me feel a bit fuller, must be all the fizz. ha ha
Hope this helps.
Cath