I think water is one of the most overlooked components when dealing with weight loss. Replacing other fluids, as Jason suggested, is a powerful tool in reducing the amount of calories consumed on a regular basis.
Beyond fluid substitutions, water does have physiological impacts on the ability to lose weight. Proper hydration is essential in one’s ability to perform optimally in the gym, or even in daily life. This is important in one’s ability to couple nutrition and exercise in a weight loss program. Not only that, the body will oxidize fat as a fuel source more efficiently when properly hydrated.
I could name countless benefits of adequate hydration and it’s importance in weight loss, sport performance, and general health. Quite possibly the only time water won’t help someone with weight loss is if they are already adequately hydrated (which a majority of the population is chronically dehydrated).