Can I mix baby formula with water?

Depends. The powdered formula is designed to mix with any approved source of regular tap water.You do not have to buy "baby water" sold in stores. If you are comfortable about the safety of your home tap water it is fine for formula use. Ready to feed formulas should not be diluted with water. If your area or house has rusty pipes and questionable water, use bottled water instead.
Only... ...If it's powdered or concentrated, and only as instructed. Ready-to-eat formula should not be diluted any further.