I recently stayed in a golf hotel in Turkey which had a massive spa area, which also included a 'Japanese pool'. Out of curiosity, I went for a swim there rather than in the conventional pool at the hotel.
The pool was around 25m long, unheated (can't have been more than 18 degrees C) with uniform depth and had wooden sides, which felt quite slimy, like there was algae present, but the most noticeable thing was how dark it was. It was dark outside, but the lighting in the pool was very subdued, and I could barely see where I was going. The only other person in the pool was a woman who I saw entering, but could hardly see her when swimming. Though we did manage to stay some distance apart so there was no collision.
Anyway, it occurred to me after my swim that I could have been naked, as no one could have seen whether I was wearing Speedos or not. The only problem would have been exiting the pool into the lighter poolside area, but I could have said that I was following Japanese customs....