In Geneva, apart from being able to swim in the massive Geneva lake (100km long, crystal clear like most seas), one can swim portions of Rhone that flows out of the lake. The only exceptions are places where it can drown you or chop you up quickly due to some water constructs (for lack of better words).
Its also quite popular to hop on some inflatable boat or mattress once its safe, just before Jonction [1] which is amazing place on its own, and just take the slow flow for couple of kms in very nice forest environment. Basically till next dam.
Zurich has very similar situation, just way more restricted for free lake swimming.