Spluch offers this item, of a floating swimming pool in Berlin, Germany. As temperatures went to the unusually high 32 degrees (89 F) there, people flocked to ways to keep cool.
It does look odd, but one thing that strikes me immediately is that the people aren't swimming in the river itself. Despite a beach-looking area, the actual water is fenced and off-limits. Shouldn't the goal be to be able to swim in the actual river water?