Summarize:

One of the most straightforward ways to quickly lose water weight is to simply sweat it out. This is a short-term technique that is commonly used by boxers and other fighters to shed extra pounds before a weigh-in. You can make yourself sweat in any number of ways, but arguably the most time-efficient way would be to spend some time in a sauna. Here you will quickly sweat and drop water weight.  Because saunas can be intense, you should only sit in them for short periods of 15 minutes to half an hour. After each short interval check your weight to see how much you have lost.  If you sweat a lot in the sauna and become dehydrated your body may start retaining water, so have some water handy and monitor your weight loss regularly.  A hot bath will work in the same way as a sauna. An even more simple way to make yourself sweat is to exercise. If you try running, cycling, or any other kind of strenuous physical activity, you will begin to sweat which will result in you temporarily losing some water weight. Some athletes will train in extra layers of clothing to provoke more sweating, but this can be dangerous and lead to overheating which can be fatal.  Bikram yoga is an example of exercising in a heated environment that will cause you to sweat much more than normal. The heat and humidity mean there is a potential for heat-related illness and you are advised to check with your doctor before undertaking such a workout. Another way to provoke sweating is to exercise while wearing a sauna suit. These suits simply make you sweat more when you exercise than you would if you were wearing more appropriate clothing. As with all the sweating techniques, you can lose a number of pounds of water weight very quickly, but these will be regained even more quickly once you eat or drink something. With all of these sweating techniques, the dangers of dehydration, heat-related illnesses, and an electro-deficit are very real. You should speak with a medical professional before considering any of these options. If you are trying to drop weight for a boxing match or a wrestling bout, be aware that sudden weight loss can make it harder for you to think clearly, cause you to lose power, and lead to sudden mood changes.
Visit the sauna. Exercise. Try a sauna suit. Know the risks and costs.