The type and quality of water used in making pizza dough is one of those emotional issues that will never get resolved to everyone's satisfaction.
Marc Malnati likes to make a big thing about using Lake Michigan water, insisting that his pizzas can't be replicated elsewhere because of the difference in the water. The New York City restaurant/pizzeria Naples 45 gets the water for its Neapolitan style pizza dough from a source (I believe it's in Pennsylvania) that "designs" the water to have the same chemical composition as the water in Naples, Italy. Previously, the restaurant imported the water directly from Naples. Along the same lines, a Southern California pizzeria, Johnnie's New York Pizzeria and Caffe, in its efforts to introduce New York style pizzas to Californians, went so far as to duplicate the mineral content of New York City tap water to use in the making of its pizza doughs.
I live outside of Dallas, and the water here is virtually undrinkable, never mind using it to make pizza dough. One local pizza maker recently described the Dallas water as being "so hard you could break your nose splashing handfuls of it in your face in the morning." I have tried just about every kind of water in my doughs, from expensive bottled waters, sparkling and non-sparkling, imported and domestic, and have not been able to detect any difference. I now just use the inexpensive jug water. The label says that it is municipal water treated by carbon filtration reverse osmosis, ultraviolet treatment, microfiltration, and ozonation. The spring water version I use is treated in the same way but without reverse osmosis. I suspect all of this treatment robs the water of some flavor enhancing components and may even affect the fermentation process. But that's the way it is.
Tom Lehmann, the dough expert at PMQ, gets a lot of questions about the so-called "water myth". He believes it is just that--a myth. For a more detailed explanation, see http://www.pmq.com/cgi-bin/tt/index.cgi/read/10838.
EDIT (6/8/14): Since the above link is no longer operative, for a later post by Tom Lehmann on the subject of "water is a myth", see http://thinktank.pmq.com/threads/ny-style-crust-in-florida.679/#post-3210