An outboard motor is provided on a hull and a steering system is connected electrically to the outboard motor and adapted to steer the hull by rotating the outboard motor. Steering motors are provided to the outboard motor. A control unit has a motor selector for choosing which of the steering motors is used to steer the outboard motor. The control unit stores motor characteristic data about the steering motors and correction data based on factors that change characteristics of the steering motors. The motor selector compares a detection signal of a steering torque detector of a boat with the motor characteristic data and the correction data in order to select at least one of the steering motors to be operated.