Swimming and Your Teeth:

Large, commericial swimming pools which are sometimes chlorinated with chlorine gas rather than hypochlorite, unless carefully monitored, can quickly become highly acidic.  Improperly maintained home pools, where chlorine and pH  often fall out of balance, can also become highly acidic.  Swimming in an improperly balanced pool can cause rapid, irreversible, tooth erosion with dark tooth discoloration and sensitivity.  The link below will take you to a recent case report from New York University.

http://www.nyu.edu/about/news-publications/news/2011/05/23/is-your-swimming-pool-safe-for-your-teeth.html