The holy Ganges River is holiest in Varanasi. And therefore, taking a dip in the river is a spiritual experience.
It might be spiritually pure, but it's not ecologically pure, by any means. The city of Varanasi alone deposits 200 million liters of untreated human sewage per day. Then there are the ashes from the 30,000 cremations that take place on the river annually. It's like a cocktail for various water-born illnesses.