Lots of shoreline swimming areas are very toxic. The article at JonBarron.org contains many disturbing facts about bacterial contamination at USA beaches. Such pollution has led to so many beach closures that they even surpass beach closures for oil spills and those which were a result of hurricanes. Even high-class tourist spots have beach closures for bacterial contamination. A rule of thumb for your safety is to check beach closure reports. Another is to always take your swim as far as possible from the juncture of where the river or stream empties into the lake, bay or ocean.