When the thermometer get an heatstroke it's nice to be able to jump into the water. Luckily Berlin is surrounded by lakes and other Fribäder [https://www.berlin.de/special/sport-und-fitness/schwimmen/schwimmbad/kategorie/freibad/] within a reasonable travel distance by bike and/or public transport. If the lakes are easy to reach, they are some efforts to provide to get places, water quality and sun or shadow. A short review of my last visits below.
Water so so, small
A big sausage fest
not much sun
but view on Teufelberg
Relatively big, a mix of locals, families and people coming from far. As you walk away from the U-Bahn you get more easily a tiny spot. Plus you can jog around and the path is almost exactly 5km.
No naked people, water is not transparent but quality ok. Getting less full as the evening comes, leaving only families and a nice atmosphere.
The best, only the best.
You have to deserve it and cycle a lot even when doing a part by S-bahn.
Superb water quality, beautiful green color. There is an island!!! There is bier garten on the island
There is a tiny lovely ferry!!! They are fkk people, but on the island they aren't so ugly.
As the other lakes there are not boats with engines, making it safe to swim. And I recall, the water quality is great. So a bit if an expedition but worth it.