Your metabolic rate can be several times higher in a pool or other body of water than it is on land without any exercise at all. Body heat is lost at a much higher rate when you are submerged in water and your body will burn more fuel to maintain core temperature.
This is something fighters do when they're going extreme weight loss before a weight in. I'm addition to dehydrating themselves and eating clean, they have several different cold exposure methods that causes the body to burn more calories.