Hypothermia...
I am seeing a show on the discovery channel right now that is about how easy it is for a person to get hypothermia in the water. They showed how even in what would be considered warm water when you get in your body heat dramatically drops.. which makes sense because even when going into the swimming pool when you get out you are shivering. I'm reminded of the Google maps suggestion that in order to get from New York to France one step is to swim across the ocean. I told people the few who have done that had special conditioning so they can swim across water of low temperature such as that. An average person can only swim maybe an hour at the most in that water before hypothermia sets in. That's also not to mention other hazards that are in the ocean. =P