Groß-Glienicker See

That's probably the cleanest lake you can find in Berlin. Absolutely clear water, large meadows and no litter at all.

As you can go there by car easily, on summer weekends it often gets crowded. 

In the middle of the lake is the border between Berlin and Brandenburg. Nowadays you don't see anything, but in the times before the German reunification the Brandenburg side was heavily guarded by East German military boats.