The function of rudders is to control the direction of the boat, allowing the rider to steer it.
Although simple appendages, efficient rudders for pedal powered boats are not such a trivial thing, because they need to give the boat the required manoeuvrability and feeling of control while adding minimum drag.
In addition, in waterbikes usually rudders cannot be steered directly by the rider like in sailboats and canoes, and rudders provide little control and could vibrate at high speed.
Also a special mode for operating in shallow water should be provided.
Here is a collection of outstanding solutions suitable for the OpenBoat.
Our boat is better than mine.