Sparkling water dehydrates the heck out of me. Seltzers and mineral waters etc. are worse in that regard than even something like Coca Cola, I think because they tend to be more carbonated.
Some soda water is salty (maybe tonic), but in general it's either just carbonated, or it's carbonated and has a bunch of minerals in it (more expensive mineral water that might have calcium, potassium, etc. in it).
Certainly symptoms like dry mucous membranes or lightheadedness can be caused by mechanisms other than dehydration, and it would surprise me if sparkling water could cause symptomatic dehydration (as opposed to correcting it, since the main ingredient is water).
I have no idea why so much sodium is present.