Well, if you are drinking a lot of water and eating a lot of unhealthy food, then no. But if you are drinking JUST water then yeah you would lose weight. But if you are just fitting that into your regular diet, then probably not.
A sure way? No...But it is a factor. It makes you feel fuller .. helps to curb your appetite and hydrates you...which also helps to reduce cravings. Reducing your caloric intake and increasing your activity is what causes you to lose weight.