Well now I feel compelled to make a Gothenburg-centric remark, ha ha.

I literally swam not far from the archipelago pier last weekend, and there are many swimmable lakes around the city. Since we're talking about swimming in the city, the best option is likely the brand new public spot in Frihamnen [1].

[1]: https://www.goteborg.com/en/places/the-public-pool-in-friham...