In Stockholm, you can swim in most of the city center - except where the large ship lanes are. Thousands of people out swimming on a hot summers day.
* Farmers having runaway water with lots of nutrition makes algae bloom * Hotter summer also lead to more algae * More unstable climate with flash rain makes sewage seep out into the fjord when the water handling stations can't handle the flow * Fishing boats are destroying the sea bed
If this isn't turned around, our nice fjord will soon be dead, and possibly not safe to use.
Not that fertilizer isn't making things worse (though sometimes it makes things better - farmers don't want their expensive fertilizer to wash away and so look for ways to keep it on the field), but it isn't the whole story.
Living on west coast of Sweden, our family went to Stockholm this summer and as the temperature rose, we asked were to go and swim and basically no one knew and we didn’t see people doing it either.
Many places in central Stockholm have been built out from natural beaches by claiming water so the water will be deep at those points (probably held quays that was used for transports before trucks took over), but even in many places like that nobody would bat an eye if someone took a non-nudist swim.
Apart from the extremes most local teens and upwards will find cliffs by the water, as mentioned not appropriate for smaller kids and seniors as it's adjacent to deep water but perfect otherwise.
Most areas are considered appropriate for most of the summer in the inner city, while lakes are everywhere outside the small centre, and quite many are private enough that you can find spots to go skinny dipping without a second thought.
[0] https://www.havochvatten.se/badplatser-och-badvatten/kommune...
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...