My vet's response:
"Cats are metabolically really weird animals. We used to think cats were really sensitive to things in their diets, which would cause them to form crystals in their urine.
Turns out, it not generally diet. If a cat is under stress, it can have a weird cortical response which lowers the pH of the urine and allows struvite crystals to form. Not a big deal in female cats, but male cats have a much longer and narrower urethra and the crystals and resulting mucus can cause a urethral obstruction than can cause an immediate and life threatening crisis.
In short, water is not a big deal. Tap water is fine. High strung, anxious kitties could benefit from a low acid diet. I give you permission to forward this to any of your friends."