The short answer - yes, you can lose a LOT of water weight overnight.
However, as I have said before, losing water weight is no joke, it is dangerous!
When you lose water weight, you also lose electrolytes and if you're elctrolytes become unbalanced (which happens), you are at risk for heart attack.
Don't do it, it doesn't feel good anyway. I made the mistake once when I was competing in bodybuilding and I'll never do it again.
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.