Electrolytes. Sodium, potassium, magnesium.
Also, food. You shouldn’t feel more than possibly brief moments of nausea. If you are, it’s probably because of electrolytes. Eat, feel better, fast again.
I’ve honestly popped a tums before, the chewable kind. I’m not a 100% purist when it comes to water fasting though (I enjoy teas and the occasional black coffee)
Dr. Fung also talks about sometimes adding a little lemon to water which sounds counterintuitive but works.