Wannsee
Arguably the most popular of Berlin's lakes, Wannsee is actually compromised of two bodies of water known as big and little Wannsee. The popularity of this lake is largely down to its long beach which stretches for over 1km alongside the clear waters. There are facilities where you can purchase cold drinks and snacks, as well as a nudist area for those feeling adventurous. It is located to the west of the city and is easily accessible from S-Bahn Berlin-Wannsee.
Wannsee is situated close to the historical town of Potsdam, so you could easily combine a trip to the popular tourist town in the morning with a visit to the beach in the afternoon.