However as you lose fat, your body can replace the fat with water and your weight wont change.
If you combine fasting with intermittent dry fasting, you can stop that happening too.
Strictly speaking, you don't need much water while fasting/in ketosis as long as you have some body fat. The human body is utterly amazing in its ability to convert fat into water and a source of energy.
While dry fasting, I get a real kick from seeing my body turn fat into pee. There is no faster or healthier way to lose fat. Katabolic water is also the purest form of water there is. The absence of any thirst after 5 days (or more) of not drinking any water is even more amazing. It is totally counterintuitive, and contrary to everything you have ever been told about our ability to survive for more than 3 days without water. That might be true during normal feeding, but most emphatically not during fasting.