I grew up swimming in our local river. Obviously the river Thames is a different kind of beast with currents, etc.

However I nowadays would be very reluctant for swimming in rivers as soon as one is downstream of a sewer leading into the waters.

Furthermore the river I swam in is downstream of an area that industrially was heavily involved in galvanics. While the water itself is clean, who knows what heavy metals are still in the river bed and mud.

I can only imagine what the mud of the river Thames and its sources contains.

Oh and even in the mountains I remember how - on a hike - a friend contemplated drinking from that clear water stream shooting down the rocks and was glad not to have done that after seeing the poop from geese and cattle just a 100 m upstream.

Amoeba, E. coli etc are not fun.