It is horrible. Sometimes the team can feel sick or weak by the end of a practice. We're talkin' school sports, not the Marine Corps. From your description, you and your teammates are exhibiting signs of dehydration. By the time you are thirsty, you already need fluids.
Water is vital to life for many reasons.
You need more if you exercise and sweat. The good news is that any non-caffeinated beverage counts, too. Any difference reflects your fluid loss from sweating. Even a modest 2 percent loss of body weight results in impaired sports performance. Once you lose more than 6 percent of your body weight in sweat, you risk dizziness, slurred speech, mental confusion, increased pulse rate, and other signs of heat illness.