I used to do water fasting 1-3 days a week for a while (a religious stint in my life). It certainly created more discipline in my life and caused me to be quite mentally focused, as I didn't have much caloric energy to gain from. I had to push pretty hard to get throught the second or third days.
I've also done similar fasting, avoiding almost all food but a few I selected for the fast. These times were just as mentally intense, but I had more energy and didn't risk long-term health as much.
I think for long-term health, it's better to get some nutrition daily, especially protein and some vitamins and minerals.
This may go against the grain of what others recommend, but I think eggs, especially raw, are a good element to include in a fast, with some citrus. Or blend some nuts and fruits and vegetables.
I think water only for an extended period of time borders on superstitious nonsense.
Thanks for entertaining my opinions.