Boat on the river -
Why locals love it
If you can't swim IN the river, how about swimming ON the river? Exactly the thoughts of the people who created this - frankly speaking VERY hip - location: A swimming pool on a boat on the river. Yep. Water surrounded by water.
Why you should visit it
Apart from the fact that it is simply cool to swim or splash around a swimming pool watching ships sail by, you'll hardly get any better impression and selection of the crazy folks that live in Berlin. They'll all be there.
There's also a beach and a sauna.