Basically seltzer water is carbonated tap water, with (maybe) some minerals/salts added.
This Article suggests that it's ok for, ie. the carbonation. I don't think carbonation can be too bad for you. Albeit, it may interfere (slightly, or negligably) with the oxyegen in your system (since you exhale carbon dioxide and inhale oxygen) So why take in more carbon dioxide if your body doens't want it?... but this is probably of no consequence. Carbonated water is water containing carbon dioxide, that's why it's all
bubbly 
So, apparently,
flavoured selter water has just natural fruit juice added. Or it could be some kind of sweetner, as suggested
here. I guess it doens't matter that much though.
That said, i imagine you'd save a lot more money just drinking ordinary water, or making your own flavoured seltzer water, by buying bulk carbonated water? (assuming you're getting small bottles of the stuff)
So not too much of a health issue really, but try to get "ordinary" water as well. You'll save some money as well
