Rio Tinto
The Rio Tinto in Spain may look visually interesting but has received its unique red color from being polluted. Runoff from a local mine contributed to the pollution of the water, and not only that, the water became very acidic. Swimming in this water will cause your body to dissolve, so definitely stay away.
The Ganges River
The Ganges River in India was affected by industrialization, as it became highly polluted and fell prey to carbon emissions. The water has become quite dangerous for swimming. The toxic river has been so polluted with garbage and plastic that is has become quite dangerous.