Check out the most polluted river in the world, the Citarum River in Indonesia. Basically used as a sewer, landfill, and chemical treatment plant (although the river provides no chemical treatment). Residents and industrial plants send their untreated waste of all kinds directly into the river. Scavengers try to pick out garbage that may be worth money if recycled.
And yes, kids swim in it.
A cleanup effort is underway, but WOW.