An ancient method of keeping a raft manoeuvrable is to draw a chain that touches the ground in order to move slower than the water, and using a huge double rudd

An ancient method of keeping a raft manoeuvrable is to draw a chain that touches the ground in order to move slower than the water, and using a huge double rudder. The raft can then be moved to one shore ore the other like a cable ferry. As a bonus, with a long chain that can be pulled in or let out partially, you get variable delta v. Another possibility, since they were two people, would be oars on the stern and bow of the vessel which allow to move orthogonally to the river flow.