American living in Copenhagen here. We check all of these boxes. The entire channel that runs through town is swimmable thanks to decades of work cleaning it up. On a warm summer day, you could expect maybe 1000+ swimmers in the water in the city centre across all the locations. We also do Vinterbadere (Winter Bathing) throughout the year. The only time you can't swim is after a day or two of sustained rain and there…
Although the outdoor swimming facilities are amazing, the swimming pools used for sports swimming are still few, small and generally overcrowded.
Which is to say anywhere pools are the primary place people swim, they will be few, small, and generally overcrowded (and also expensive). If you live where there are natural places to swim you will generally find it better, but many places don't give you those options.