More than half the content of the cells in your body are composed of water. If your water consumption is too low, your energy level will drop. Without it, you'll die. Water is so vital to enduring hot and humid weather that it seems wrong to call it a supplement. An essential would seem to be a better word.
But chronic dehydration is extremely common, particularly in the most intense environments, so supplementing would be an improvement. Adults need approximately 67.6 ounces (2 liters) of water per day. In muggy weather, the requirements go up. You can easily gauge if your body needs more water by examining the color of your urine. If it's clear, then you're treating your body right [source: Edwards].