Actually I read that water increases fat metabolism. Another thing is that water has zero calories, and because of that the process of fat burning WILL NOT slow down. It could slow down if you drank Gatorade though, because it has sugar in it, and that will cause your body to burn sugar from Gatorade, instead of your fat reserves.
Moreover, dehydration is very dangerous, so try to avoid it.
And B Fold said it right above: your weight should stay about the same before and after workout.