Many (Most) of the rivers and lakes are swimmable in Switzerland, the standards for water are incredibly high, more dangerous is not knowing the currents.

Going down the Aare river from Thun/Uttigen to Bern is a hugely popular activity to do in summer, but every year a few people drown. Even so, I'd recommend it.

https://madeinbern.com/en/stories/along-the-aare-river-2#:~:...).