There are limits to how much remediation you can do to river water quality. Not that people shouldn't try, but if e.g. upstream sites leach heavy metals, or biological contaminants deep in ground water, there's only so much you can do.

Brisbane (where I live) the river is affectionately called 'the brown snake' mainly because of silt. Even though sand mining/dredging has ended, the river is silty still. Is that why swimming isn't recommended? No: its because upstream on oxley creek, major industrial and agricultural runoff issues during historic floods in the last 10-20 years have raised contamination, fecal coliform and other problems such that you risk serious antibiotic resistant bugs if you accidentally ingest too much, or get it in cuts (I know because I row, and a rowing colleague wound up on IV antibiotics with amputation being discussed from a deep cut)

When it floods in brisbane, associated creeks continue to leach the effects of 100+ years of industrial waste and effluent into the water. Nearby cities have land which is capped 2m deep over what was septic tank discharge, it is said that shigella is still a significant risk if you dig into the soil. I don't know, I thought it died, but perhaps it persists?

Well.. that and the bullsharks but we're talking about pollution.

Yes, the Thames is coming back to life and significant fish numbers have risen in the last decade, we're miles off "the great stink" which caused Bazalgette to construct the sewers on the riverbank. Is it always entirely safe to "be in" -well no. Sources upstream occasionally dump sewage overflow along the length of the Thames. Its meant to be emergency only, but sewage overflow has to be dealt with pretty rapidly. They try not to. What do you do, when the system is overloaded?

I am amazed Paris has been able to restore river health enough to do this. Well done!