Any weight you lose by water fasting is just going to come back when you end the fast. And you can't fast forever and still expect to live. There are healthy ways to lose weight that actually work, but it's through moderation and the progress is slow.

It's been shown that dieting and fasting can mess with your "set point", your natural weight, and make it higher - your body fights to maintain your set point and you don't want to mess with it, because then your body will naturally return to higher and higher weights. That's why yo-yo dieting never works.