My doctor told me: Everything we eat has water in it. We get a lot of water from just normal eating. 2L a day is what you need, but that includes the water you get out of your food. So drink when you're thirsty. Drink when you feel like it. That's all you need to do.
I drink water a lot though, cos I drink it when I get hungry. I also like the feeling of flushing my body. The way I think about it is that the fat that you "burn" turns into the yellow stuff in your wee, and when you go to the toilet that's how you get rid of it. It dissolves in water, that's how it gets from your cells through your blood to your kidneys and then bladder. So the more you drink, the more of those little soluble bits are going to get picked up and taken away.
You've also gotta remember though, that every single substance in the world is toxic if you eat to much of it, and that includes water. If you drink too much, you will die. (Too much is quoted as somewhere between 5 and 20L, depending where you look.)