If you only drink water and dont eat will you lose weight?

If all other things in your lifestyle (food intake, exercise) were to remain consistent and we assume your current lifestyle had you at a steady weight (it wasn't causing you to gain a little weight every day) , then yes, substituting water for caloried soda would result in weight loss. That's a bit of an "if"- maybe you'd tend to eat more sweets if you weren't drinking soda, but if everything else stayed the same, you'd lose weight. Water is much better for you. You lose weight when you burn more calories than you consume. Calorie-free soda is less of a weight risk, but it can still cause some weight gain through other mechanisms.