Unfortunately, the bodies are not always thoroughly burned before being tossed in. Fuel is expensive, you know.
I used to know a fellow (now deceased) who was a veteran of the China-Burma-India theater. The only war story he told me was of when his unit boarded a troopship anchored in the Ganges at Calcutta to head home. He said that the only water the ship took on in the Ganges was for engine cooling...he and his men were on emergency water rationing from the moment they boarded the ship until they were out in the open ocean. Even at that time, the river was so polluted that it would have wiped out the ship’s water distillers within minutes.