Drinking plenty of water is crucial if you're trying to lose weight. It helps suppress appetite (we often mistake thirst for hunger pangs), so you're less likely to overeat. On the other hand, being dehydrated can result in reduced motivation, increased fatigue, and can make exercise feel much more difficult. It also impairs physical performance, heart, liver and kidney performance, and basically makes anything to do with your body more taxing. Try to get at least 2 liters of water per day.