Would you do it?
https://www.cnn.com/2023/07/26/sport...ntl/index.html

I normally prefer Paris in the Fall, but I can't imagine anything better than sunbathing on the banks of the Seine after a nice refreshing swim in (hopefully) sparkling clean water. May have to start thinking about summer visits!

People reacted that way when the Bassin de la Villette was authorized for summer swimming a few years ago. The water quality is checked twice a day. The first photo in the CNN article is of the Bassin de la Villette. It is definitely no worse than swimming in the ocean.

If it wasn't safe they would leave it banned.
Swimming water quality is frequently and regularly tested.
Swimming is allowed in lots of rivers in Europe, but I think you have to be a good swimmer to do it, it is very different to swimming in a pool, or the sea.

I've sailed through there and there are some very strong currents due to the banking forcing the river into too tight a space, take sensible precautions

My observation is that it is less swimming than floating on the river, at least that seems to be the case in Basel and Zurich. Years ago my daughter joined her Swiss cousin during a long overlay in Zurich for a swim down the river, and she is not a strong swimmer. In Basel I noticed that the swimmers stayed close to the right bank; might there be less current on that side?

In Paris it will be swimming, certainly for the Olympic events taking place in the river.

Leisure swimming after the Olympics will almost certain be confined to basins like in the Bassin de la Villette. They can't risk people swimming into the middle of the river with all of the boats.