With the passing of Ken Venturi, film clips show his dehydration at the '64 Open nearly cost him his life. Not sure whether or not he was drinking fluids during those 36 final holes or not. It was a dangerous situation. Electrolye loss is serious. There are plenty of products listed above by othersthat are far better than just water consumption. As a Tennis Pro, I've used Clif Shot mix during 3 hour tournament matches in 100+ temps and never had a problem.
you're right about electrolyte loss, but typically water is all you need. i've always gone by the rule, you only need electrolyte replacement if you are doing an intense physical activity for an hour or more. as a runner, some of these electrolyte drinks actually do more harm if you don't "work hard enough" first. for just an ordinary day on the course, water will give you the exact same benefits. now if it's 100 degrees and you are walking, that changes things.