There's lots of still water in shot; the area seems flooded, so maybe they flooded it for the film (or it's a place that the river normally floods into during the wet season). Maybe sending pollutants downstream in a river (there must be many rivers, like the Thames, that are not utilised in the food chain) is not as directly life threatening as paddling in that water. I think there are many places (the stagnating canals in Ghent come to mind) where falling into the water requires treatment for toxins before all else.