Apply some sort of creamy conditioner or oil to your hair when you style it that day, then just prior to going into the pool, wet your hair with clean water. Since you mentioned, a water park, I doubt you'll want to wear a swim cap - if you do chose this option, then instead of wetting your hair, apply petroleum jelly or a conditioner around your hairline and put on the swim cap. This will help to prevent your hair from absorbing the chlorinated water.
After you're done with the pool, immediately wash your hair with shampoo to remove the chlorine and apply conditioner. Some people recommend a chelating shampoo, but I've found that any clear, SLS based shampoo does the trick. This is the most important step so even if you don't do anything else, make sure that you wash your hair as soon as you can after being in chlorinated water.
ETA this advice also applies to salt water for those who go to the beach.