For information on organic risks present in recreational water, please reference the following information from the Centers for Disease Control and Prevention:

Healthy Swimming/Recreational Water
http://www.cdc.gov/healthywater/swimming/

Water and Parasites
http://www.cdc.gov/parasites/water.html

Naegleria fowleri - Primary Amebic Meningoencephalitis (PAM)
http://www.cdc.gov/parasites/naegleria/general.html