Studies show fasting for water more than 24 hours does nothing for you, up until that 24 hours it's great for you but past it no observable effect has been made and they've studied it at length.
I use water fasts because when your body has no water it has to go searching for it, meaning it will begun breaking down fat stores which means I'll be able to get into ketosis faster, but also breaks down muscle, but when you fast without food for up to 2 days you will get a big shot of HGH (human growth hormone) which helps to defer muscle loss during this process, but that HGH protection only lasts for 2 days so if you fast for water past that your body is breaking down muscle as well as fat to meet the water, likely even dually so if you are using creatine but I don't believe there is any actual studies on that other than the fact that creatine makes your muscles hold more water.