It has been discussed every now and then, but it's always too complicated, or the solutions available aren't liked by the people in charge.
https://de.wikipedia.org/wiki/Flussbad_Berlin
https://www.flussbad-berlin.de/
https://www.visitberlin.de/en/badeschiff-der-arena
"Our river is so dirty, it's too dangerous to swim in there. Let's build a swimming pool into the river!"
It has been discussed every now and then, but it's always too complicated, or the solutions available aren't liked by the people in charge.