Some last random thoughts then (hopefully) I’m done with the topic.
One choshuve talmid chacham was aware of this historical fact. He told me that it wasn’t about levity. They didn’t have plumbing. And the river was a way to cool off.
I found some sources from two hundred years ago, about specific hours that the men would stay away from the water sources. But these were all in small shtetlach.
Ujm, maybe they weren’t observant. I don’t know. My point is that they weren’t MO. Maybe what happened was that the observant copied the non-observant.
Avira, the Steipler’s letter is later. It was a major issue then. And not just MO. It took around ten years to get it under control. You see, sometimes breaches in Torah can be contained.
Lakewhut, so MO has a rabbi problem. Mixed swimming is a casualty of that. A yeshivish person who insists on mixed swimming or some other issur will go somewhere without a real leader of a rabbi. Last generation, that was MO. I’m not convinced it will remain that way. Or even if it still is that way.