In my opinion (and I know other's will disagree), plain carbonated water is the equivalent of plain water. If it has added sweetener or salt, that's another story. I personally love seltzer water with a squeeze of lemon or lime and I think it does just as good a job of keeping me hydrated as plain water.

The best rule is always to listen to your own body, because some of these things work differently for different people. Does drinking seltzer water instead of plain water make you feel any different or does it interfere with your weight loss? IMHO, if you answer "no" to those questions, you are probably fine with the seltzer water.