In my experience, you gain a lot of water weight back immediately after you start eating carbs, but water fasting allows you to reset your diet if it was bad before. If you follow a clean diet, the body fat that you burned off after the water fast will stay off.
I lost 10lbs in 5 days overall. Slowly crept back up over time but it’s been nearly 6 months now. Still net 4lbs lighter but planning another longer fast before summer starts properly. Eat more than you need and you’ll put the weight back on and vice versa.