If you're going to exercise at high intensity for long periods of time, then consider sports drinks. Even during water workouts, your body loses valuable electrolytes and heats up. Caffeine and alcohol can dehydrate the body further, so avoid them when outside in hot weather. Always wear sunscreen because a sunburn decreases your body's ability to release heat, and when in doubt, take it inside. Call it quits if you experience weakness, headache, dizziness, nausea cramps or a rapid heartbeat. These are all indicators of heat-related illness.