I don't know about water fasting. There's no nutrition in water. I did do a vegetable juice fast for three weeks once, lost 12-13lbs and my skin looked perfect afterwards. It makes sense that in order to rebuild damaged tissue, your body would want some potent nutrition fast-tracked to it, so that's what I did.
Well yeah, enriched nutrition is great for your body, but water fast is not about nutrition per se. It's about across the board rest you provide for your body. It means your body switches to finding "nutrition" inside you (consumes fat, toxins, parasites, dead cells, damaged tissues etc.).
Every organ (skin the largest organ), cell, tissue etc. cleanses and regenerates.