I'm not sure about other cultures, but in Japan they're pretty explicit about washing before entering an onsen. They even discourage putting your towel in the

I'm not sure about other cultures, but in Japan they're pretty explicit about washing before entering an onsen. They even discourage putting your towel in the water, wearing a bathing suit, or submerging your head (as well as going when you have open cuts or sores). But, they also prefer mineral water to chlorine and it's generally fed from a spring or the water is changed every day. I'm not a fan of overly cleanliness, but not doing these things means you have to add more chlorine. In addition to the aesthetic unpleasantness of chlorine and other chemicals, it's harder on the pool equipment and like another commenter said, it should require better ventilation that most pools currently have.