I was in Paris a few weeks ago after days of no rain and stood watching the river from a bridge. It looked absolutely filthy - very grubby, with the occasional piece of rubbish floating by (pretty much what you would expect a river in a city to look like). And that's before adding large amounts of rain (which must increase sewage and farm waste flow into the river substantially). I suspect the river may one day be clean, but I think there's a lot of work needed upstream before it's ever going to be in a suitable, safe state to ingest while swimming.