Water is needed by the entire body to keep it running effectively. I don't think that anyone has proven that increased amounts of water will increase fat metabolism or removal of waste products. The only thing that will increase fat metabolism is increased activity. I could weight 500 lbs and drink 2 gallons of water a day and not lose weight. I will likely have increased activity by having to walk to the bathroom more.
If you are like me you've read a zillion books on fitness and most of the authors can't agree on the subject of water. Most say to drink at least 8 glasses a day, more if you exercise a lot because you are losing fluids through sweat. Frankly from some of the articles I've read about the subject most people don't even know where the 8 glasses a day recommendation originally came from. Everyone seems to stick with it though.
I believe that there is an article about water in the Library section in the general forum that is very informative.