Water fast meaning you live on just water? In what way does that benefit one's health? (I'm not looking to get personal.) Also, doesn't that make have to use the restroom every few minutes?
Yes to Q1.
Q2: extended fasting starts a bodily process called autophagy, where the body destroys it's own damaged and sick tissue. It might be able to help reset gene expression.
Disclaimer: don't try it without medical supervision or doing the necessary research and as within halacha stop fasting if you feel really unwell...
Q3: Sodium on it's own would do that. But not in this ratio with the potassium.