Your body is 3/4 made up of water. Water that you drink (especially in water fasting) cleans out all your organs, fatty acids/tissues, etc. But, as it travels through your system and picks up toxins and such, it needs to be filter out by urination and sweating. Your body uses up so much water during fasting so there is a high demand for it.
So, you're on the water fast, drinking about 2 liters of water per day and you are filtering it out through the excretory system much more frequently than normal.
The answer is that you are constantly losing water weight if you drink a fair amount of it. The number of oz your should drink is based on half of your body weight. So if you were 150 pounds, you would drink 74 oz per day.
To lose more water weight, you would have to drink more water and/or exercise to sweat the water off. But exercising during Water Fasting isn't really required because this is a time of rest and you need to conserve your energy.