When I had pet triops (very sensitive to water) we had them on distilled water, it's not overly expensive and just needs storing, I'd imagine this would be safest.
Cagedbirdsinging is right too, most bottled water is just tap water that is filtered and re-labled, and the chemicals that get into it through the plastic bottles aren't amazingly good for you either.
That being said, my ratties are just on normal tap water.