Our bodies are largely composed of water, and need a constant supply to function efficiently. If you don't drink enough, your body uses the water it gets for essential processes (for example, staying alive!), and stops non-essential processes - fat burning for example!
Help the liver! The liver is a wonderfully flexible organ - it works to convert stored fat into energy, and also works as a back-up for the kidneys when they are struggling to cope. The main reason that the kidneys struggle to work properly is, you guessed it, lack of water. So if you don't drink enough water, the liver kicks in to help out the kidneys, diverting it from its primary function of metabolizing stored fat.
Thermic cost: The Thermic Effect of Food (TEF) refers to the amount of calories that it takes your body to digest, absorb, and metabolize any food or drink that you consume - this typically makes up about 5 to 15% of your total daily calorie expenditure. Interestingly, drinking cold water increases the TEF - this is because in order to process it, your body has to raise the water temperature to your body's temperature. However, as water has no calories, your body ends up actually burning calories every time you drink cold water.
The bottom line? Anyone wanting to lose weight needs to use everything at their disposal, and drinking water might be the simplest, cheapest way yet to boost your slimming success rate.