And just to add to the confusion a bit more.... as someone mentioned before about soda water's sodium content - some bottled mineral waters have got quite a high sodium content too.
Evian has the lowest with 5mg/litre
Highland Spring has 9mg/litre
Volvic has 11.6mg/litre
Tesco own brand has 30mg/litre
and scarily
Nestle Purelife has 40.4mg/litre
Anything less than 20mg/litre is considered low sodium. So it's worth checking the bottles.