I WAS SITTING in a tidal pool on the cold, windy beach of an ancient volcanic island 60 miles off Japan's southwest coast. The ocean pounded the beach's rocks a few feet away. The lean Japanese man sitting next to me had left his bike shorts in a pile on the ground behind a seaweed-strewn rock, just opposite my discarded clothes. My fellow bather and I shifted our gazes between the oncoming waves and one another. It was all a bit awkward, but neither of us budged.

We were in one of Japan's celebrated onsen, the natural...