Most people can live without food for a month. Without fluid, 3 days. Sodium and potassium help you retain fluid so if you get all three, you will survive until you recover. It used to be when IVs where the only option, people would die from dehydration if they didn't get IV treatment. Pedialyte is a combination of salt, potassium, sugar and I think baking soda and it's a magic rehydration formula that has saved many lives and cost pennies a dose as opposed to many dollars for IV treatment. Gatorade is a close approximation and many people have it laying around the house.
In short, you need to retain fluid and a rehydration fluid will work better than water alone.