Normally water contains ions. If we add this normal water into a battery, then the ions can clog the electrodes by forming a chemical reaction. They do this by forming a layer that blocks chemical reactions and thus prevents the generation of electric current. Other than ions, normal drinking water might also have organic materials or dust particles which can hamper any chemical reaction. So it is always advised to add distilled water since it does not have any ions and will ake sure that the battery operates smoothly.