Is it healthy to fast with just water? And how often should it be done?

Not especially. If you are fasting for a day for religious or spiritual reasons, then you should be ok as long as you don't have diseases like diabetes or hypoglycemia. If you're trying to lose weight, this won't help as your body will slow metabolism. For bowel health is unnecessary as bowel constantly clearing things our, even during sleep. See your doc for specifics.
Not recommended. Water is "necessary but not sufficient" as taught in training! maintenance fluid intake should contain minerals, vitamines, and some basic fuel for energy that is essential for many cells to function best (especially simple carbohydrates and protein, even on a short term basis). Water alone provides only a portion of the needed elements for healthy cellular and body functions.
No. Not healthy. Why with water alone? You're not getting any nutrients or minerals or calories.