Steering control system for a watercraft with a steering tiller manually operated and operatively connected to a direction changing member acting on or into the water, such as a rudder plate or an outboard motor; a device locking the steering tiller in the steering position. which can be activated for maintaining the tiller in a predetermined angular position and can be deactivated for allowing the tiller to be moved in a different angular position to carry out a change in the direction. The locking device is of the hydraulic type, that is, oil-hydraulic type, and includes a hydraulic cylinder having a rod that is fastened stationary to the transom of the watercraft and a cylindrical body that is movable along the rod and is connected to a steering arm of the motor or of the rudder member, or vice versa, there being provided a closed circuit for flowing the oil between the two chambers of the cylinder, within which circuit there is provided a valve opening and closing the circuit, which locking device can be activated/deactivated by an actuator that opens/closes the valve opening and closing the circuit. The system further includes an interposition element between the steering tiller and the watercraft direction changing member, the tiller being pivotably articulated to such element, and a member transforming the pivoting movement of the tiller with respect to the interposition element into commands actuating the valve.