Saffa-Insel is a little island in Lake Zurich, accessed by footbridge from the Landewiese park. In summer, lots of people come here to sunbathe and swim since it's free to use and such a pretty spot along the lake.
The entry to the water is a little sketchy, sliding over rocks. Then you have to be careful to not swim in the path of any boats. Kids like to jump off the bridge, but careful as it's not that deep.
There are public toilets nearby, but no changing rooms or cafe. If you need more services or have little kids, you may prefer the Mythenquai swimming area just north this park, which has an entrance fee, cafe and safer areas to swim.
To get here:
The closest bus stop is Landiwiese, a 4 min ride from Zürich Bürkliplatz on bus 161 or 165. Use the nearby footbridge to cross the busy street, don't run across!
It's also near the Zürich Wollishofen train station. There is a tunnel under the busy street to reach the park.
You can also just walk here from Zürich, along the nice lakeside walking path. From Zürich Bürkliplatz, it takes about 30 mins walking to get here.