for 100% breast fed babies, they dont need water, and you shouldnt give it. There is some suggestions that it can be damaging for their kidneys. If Jazz for ff, then, abit is ok, as the formula can make then dehyrdated and/or constipated.
But, just offer her the boob abit more often. She may only take abit of the foremilk, which will satisfy her thirst. I tihnk it was more 'done in the good ol' days' type thing. Just keep demand feeding, she will only take as much as she needs.