Many years ago, I was in the Army during the Vietnam Era. While in training at Fort Polk, we didn't have any electrolyte drinks, although we were encouraged to swallow a salt tablet with our breakfast meal. In the summertime, it was very hot and very humid, but still we went out on long marches. Water management was odd. We were told to carry two canteens of water for a 15-mile march. However, we were trained not to let the canteens get partially full. Then, they would slosh and make too much noise. We could carry them full, or we could carry them empty, but the half-full slosh was bad. As a result, some guys wouldn't drink at all. As you would expect, they became the first victims of either heat exhaustion or heat stroke. On one day, we had one of each.
Now with modern water bottles and dilute electrolyte drinks, I can sip fluids all day and keep going much longer.