A load variation amount can be derived from an output of a load sensor for detecting an external force acting on a watercraft. A computation can be performed whether or not the load variation amount is larger than a reference value calculated based on the load sensor output, a running state, and a navigation velocity. The width and the magnitude of a pulse are determined based on the load sensor output, the running state, and the navigation velocity, and the pulse is applied to a steering as reaction torque.