A reaction ferry is a barge that is propelled solely by the force of the river's water streaming by. The ferry is anchored to the riverbed in one spot and the tether splits into bow- and stern- lines. Turning the helm, the length of those lines is changed and with that the angle of attack for the hydrodynamic forces changes. In reaction the ferry pivots around its anchor, reaching the other river bank.
This example here is part of a relief project- the whole equipment was built, and is run and maintained by a youth training scheme.

Bamberg