And the strong current?

The current of the Donaukanal is a risk for inexperienced swimmers. However, good swimmers can easily enter and exit Donaukanal.

As an example the river Aare in the city of Berne in Switzerland is enjoyed by thousands of swimmers on a sunny summer day – even though the current is way wilder than in the Donaukanal.

In fact being carried by the stream is a very pleasurable urban experience. It's a fully immersive experience, a way to explore the city from a completely different eye-level  than in usual everyday activities.