Why doesn't being surrounded by gallons of water in a swimming pool cause sodium imbalances and/or water intoxication? The pool water is isotonic with your internal body fluids.
It is impossible to sweat in a swimming pool; therefore, your body loses no sodium.
Your skin is not as permeable as the lining of your digestive system.
The chlorine in the pool water prevents your body from losing sodium to the surrounding environment, or Swimming in pool water can cause a sodium imbalance; this is why swimming in the ocean is preferable.