Working in an air-conditioned environment, alcohol, coffee, exercise and speech, all tax our water reserves. The more we do of any of the above activities, the more water we need to bring us back into balance. We need roughly 2 litres per day to stay hydrated. The rest should come from fruit and vegetables. If you feel thirsty, you are already dehydrated.
65-90% of human cells consist of water. Weight loss during a workout is due to lost water and needs replacing. Weigh yourself before and after a particularly sweaty workout to see what you have lost. A pound of weight loss is about 1/2 litre of lost water. A method used for extreme weight loss in boxing circles is to dehydrate the body so fighters can make a weigh in. Immediately after, they rehydrate themselves. It seems odd to potentially dehydrate a nerve cell then beat the living hell out of it for 12 rounds.
Electrolyte balance in the body is crucial for optimal nerve function. Without key electrolytes, muscle weakness or severe muscle contractions may occur. An old wives tale of adding table salt to your water isn't enough as refined salt has high levels of sodium ( we already have far too much in our modern diet). Unrefined salt has higher levels of potassium (which we need), but can be hard to get hold of. Instead, I add electrolytes to my workout water. This is even more crucial for those who like endurance events or training.
Signs of dehydration include dark urine, muscle tingling and cramps. Any more than 10% water loss can be fatal. If you like to drink alcohol, drink water between each drink and couple of glasses when you get home. You'll feel better in the morning.