How is your fast going? As for exercise, walking is encouraged. Also, you probably know this... be careful if getting up too quickly. You might feel dizzy, pause breathe deeply then get up slowly if that ever happens.
Go for it! Whatever your reasons for doing this, Water Fasts can be very healing. I've just started a long one myself.
Here is a link from this site: The stages of Water Fasting (from Chrisb1) Good Info