The idea that the Ganges River is polluted is a misconception.
This is the "starting point"
of what is known as the "Ganges River. " This is an image of Devuprayag, where the Arakanaanda and Bhagirathi rivers merge. The Ganges officially begins from this right edge. (This point is revered as a sacred site.)
Where exactly is it contaminated?
The Ganges River, by its very nature, originates from the melting snow of the Himalayas, so it's bound to be beautiful.
(In Hindu mythology, the Ganges is a river that flows through the heavens where the gods reside. A wise man in mythology negotiated with the gods, causing the river to begin flowing down to Earth. Because the impact would have destroyed the Earthly world if it had continued as it was, Lord Shiva is said to have caught it with his hair and dropped it into the Himalayas.)
(Further upstream along the Bhagirathi River, which has a Hindu origin, lies "The Cow's Mouth." What you see in the background is not a rock, but the end of a glacier, which melts along with the sand and gravel that have carved out the canyon, making the water cloudy. It tastes gritty when you drink it, but it's not polluted.)
In other words, it is humanity, with its excessive population, that is polluting the Ganges River.
Well, it's true that India's environmental infrastructure is poor. Therefore, the further downstream you go in the Ganges, the dirtier it becomes due to the mixing of domestic and industrial wastewater. (While cremated bodies are released into the water, in some cases the body is simply buried in the water, so it's somewhat inevitable that you'll get sick if you bathe downstream.)
Indians understand this very well. That's why people who have received a modern education and are conscious of hygiene go upstream to bathe. Not every Indian, Brahmin or Dalit, bathes in the Ganges River just anywhere they see it . (In urban areas, you can even buy bottled water drawn from upstream.)
If Japanese people want to bathe in the Ganges River, they should take that into consideration. They're not stupid enough to jump into the polluted downstream Ganges and get sick.