Water is all you need..IF you are supplementing your minerals through food intake. Most packaged food possess more than enough minerals to sustain you with adequate water... in areas of extreme heat or humidity where sweat is copiuos for an extended period of time, physical exertion is at max, and water consumption is high, Sodium loss without replacement can cause a serious and fatal electrolyte imbalance called hyponutremia, (not uncommon in marathoners_)
TOO much supplementation can have negative effects in the opposite way (also with potentially fatal effects_) My advice would be to hydrate primarily with water. If you have food to consume you are most likely set. If you are still concerned a packet or 2 of gatorade powder would be a good thing to have.
Rehydration formulas such as the one reccomended by WHO, are not for consumption as replacement for water.. but are rather a way to restore electrolyte balance to to people who have suffered from extreme Gastro intestinal distress, or a heat related emergency. Essentially the formula has the basice elements of what you would get in an IV if you where admitted for the aforementioned causes to an emergency room, except this is consumed orally. I don;t think its meant for a tired hiker outside of an emergency situation.
So again I reiterate, a powdered sports drink maybe 1 nalgene out of every 4,. Or follow Marcelo's well thought out forumla
but IMO there's no reason to start administering a cure, if you can keep the problem from ever happening.
"Most people are like slinky's they are pretty much useless, But it's fun to watch them fall down the stairs." - Anonymoous