A watercraft has an engine and a remote controller. The engine has a throttle valve unit. The remote controller provides a command signal indicative of a position of the throttle valve unit. A watercraft velocity sensor senses an actual speed of the watercraft to provide an actual speed signal. A control data input device selectively provides a control device with a manual mode signal and a constant speed mode signal. The constant speed mode signal is accompanied by a target speed signal. The control device controls the throttle valve unit based upon the command signal in the manual mode. The control device controls the throttle valve unit in the constant speed mode such that an actual speed of the watercraft coincides with the target speed of the watercraft once a state of equilibrium is reached. The control device starts the manual mode in place of the constant speed mode without the manual mode signal if the command signal changes while the control device controls the throttle valve unit in the constant speed mode.