One of the things that makes Munich so popular is its proximity to the lakes, in German a lake is called a ‘See’.
Starnberger and Tegernsee
The Starnberger See and Tegernsee are to the south and are natural glacial lakes. There are other small ones around like the Steinsee too. Both Starnberger and Tegernsee have bathing areas for the public usually signposted with ‘Erholungsgebiet’ which translates roughly as recovery / relaxation area.
To the north just below the most northern tip of the A99, the ring motorway surrounding Munich is the Feringasee. This is a man made lake designed for bathing, so it has lots of lakeside sunbathing space, is shallow and it also has a pub (Gasthof) on its banks. In the Summer it warms up quickly and in the winter it freezes over quickly making it a good choice to go ice skating of curling. It also has plenty of parking space, plus good access for wheelchairs.