Anna Zenonos: A lake on the S1 line of Berlin public transport S-Bahn (station Schlachtensee, then 20 minute walk). There is a restaurant at the other nearby lake (Schlachtensee for dining). Also a jogging/biking road round the lake and some wooden outdoor 'furniture' make it very family-friendly. Water temperature in March(2012), after a sunny unusually hot day, is 11 celsius. The local triathlon propably uses this lake as I was told (not sure).
Trees and branches fallen in it
Roman Contzen: There are actually two sports events taking place at the lake which involve swimming. a - the Volkstri in spring, which is a triathlon and b - the Swim & Run in autumn. Both events organised by weltraumjogger.de