A boat control system can include a propulsion unit controller, a port outboard motor, and a starboard outboard motor. The propulsion unit controller can includes an electronic throttle valve control section, an electronic shift control section, an electronic steering control section, a target control value calculating section, and a GPS receiver. The target control value calculating section can be adapted to calculate engine revolutions and steering angles corresponding to target values of the port outboard motor and the starboard outboard motor based on target values preset by an operator and values detected by the GPS receiver.