Weight management always comes down to calories you take in verses the calories you burn. Drinking water doesn't change that. But don't give up your bottle of water just yet...
It's possible that drinking more water will help you lose weight, especially if you replace sweetened high-calorie beverages or alcoholic beverages with plain or carbonated water. That is, of course, as long as you don't add more high-calorie foods to your diet.
Population studies suggest that people who drink more water tend to consume fewer calories. That doesn't mean drinking water directly causes you to lose weight, but some experts believe water consumption may affect metabolism.
It's possible that people who drink more water are just more health-conscious in general, so the water itself could have nothing to do with it.
Some experts believe that feeling hungry may be an indicator that you need more water. I'm not sure if that's true, but drinking water instead of eating a snack will reduce the calories you consume.
Increasing your water intake before meals may help you watch your weight because it takes up space in your stomach. This in turn may reduce the amount of food you consume during a meal -- at least if you're middle age or older; studies don't indicate that young people who drink water before a meal tend to eat less.
You don't have to drink plain water to avoid extra calories; there are other zero-calorie beverages available. You can drink coffee, tea or herbal tea, but remember that adding milk, cream or sugar will add calories. Note -- we used to think of caffeinated beverages as being diuretics (resulting in fluid loss) but studies show they provide plenty of water in spite of the caffeine.
Ahmadizad S, El-Sayed MS, MacLaren DP. "Effects of water intake on the responses of haemorheological variables to resistance exercise." Clin Hemorheol Microcirc. 2006;35(1-2):317-27.
Armstrong LE, Pumerantz AC, Roti MW, Judelson DA, Watson G, Dias JC, Sokmen B, Casa DJ, Maresh CM, Lieberman H, Kellogg M. "Fluid, electrolyte, and renal indices of hydration during 11 days of controlled caffeine consumption." Fluid, electrolyte, and renal indices of hydration during 11 days of controlled caffeine consumption.
Dennis EA, Flack KD, Davy BM. "Beverage consumption and adult weight management: A review." Eat Behav. 2009 Dec;10(4):237-46.
Stookey JD, Constant F, Popkin BM, Gardner CD. "Drinking water is associated with weight loss in overweight dieting women independent of diet and activity." Obesity (Silver Spring). 2008 Nov;16(11):2481-8.