I spend a lot of time in the pool. My mouth ends (sides) harden and crack. How to prevent this?

Moisturize. Due to the chemicals in the pool water, and also possibly your own skin 'makeup', you may be more prone to having the sides of your lips 'crack'. Simply using a waterproof lip balm will greatly help. You must keep your lips moisturized, as they are drying out, which is causing the problem. A dentist can also prescribe a medicine if it's a fungal infection that can often be the culprit.