The Water Only Diet is also called the Starvation Diet and is exactly what the name describes. You only drink water until you lose the weight you want.
The problem with this diet and others like it ("Cabbage Soup Diet" and many other variations), is that you cause a number of problems and set yourself up for more weight gain later.
1. Restricting calories too far slows the metabolism, so when you start to eat, your body will hold on to calories as fat in case you do it again.
2. Restricting calories too far causes you to burn muscle instead of fat and muscle is what burns calories, so you end up worse off than when you started.
3. It is not a sustainable solution.
If you're really interested in losing weight as fast as possible while keeping it off, I explain why and how my weight loss program works in my new weight loss and diet program.