http://www.peoplespharmacy.com/2007/06/14/tonic-water-eas/
http://en.wikipedia.org/wiki/Carbonated_water
TL:DR
Tonic water contains "Quinine" which is very good at alleviating cramps but not without risk as it can cause a potentially deadly blood disorder.
There are also some links between carbonated liquids and bone loss, meaning that carbonated drinks may lead to increased chances of fractured/broken bones. The scientific community is still undecided on this point however.