Weird as it may sound, but drinking more water decrease weight.  Here is the scientific explanation.

To decrease weight you need to burn fat fast. However, when fat of your body is burn it produces toxic substances which is stored in kidney  and filtered out by it.  If you dont drink water these substances are kept stored in kidney. If these substances are stored in large proportion it may damage kidney. So our body automatically slows down burning of fat so less of this substances are produced. And thus slow burning of fat causes you to gain weight.

If you drink enough water you will be filtering these substances fast out of the body so that body is ready to burn fat more quicker.