Hi JRP, it will keep the humidity levels up.... and I presume you mean 'Co2' in relation to the fizzy water.... it won't make any difference to the Co2 levels of the room so you may as well just use regular tap water.... Same goes for watering with carbonated water, it won't magically give your plant more C02 or anything (though some claim the fizzy water helps aerate the soil)
I do remember my first grows and I used to try everything, even used to make little Co2 generators using sugar and yeast.... anything to make my girls grow faster and better, but I assure you it has no impact whatsoever. There is far more Co2 generated just being around your plant and tending to it for a few minutes.
Best of luck with your first grow