Is the water fast safe? and what do you do?
I'm guessing it's a fast in which you only drink water? Fasting is usually a bad idea, because it can lead to eating disorders such as anorexia. I'd advise against it, because you may think you will lose weight, but really it will slow down your metabolism and then when you eat again, you'll gain more weight than usual. In general, try cutting as much sugar from your diet as you can. That is a good start.
Water fast which is also known as wet fasting is safe, but do not exceed a period of an approximated time of 10 days. After the 10 days, your body will start experiencing a slow metabolism. The energy will go down because of lack of carbohydrates, which a human being always needs to survive.