A boat propulsion system that easily and finely adjusts the rotational speed of a propeller includes an outboard motor including a power source, a propeller, a control lever to which an accelerator opening is input, an accelerator opening detection section arranged to output an operating amount of the control lever, a sensitivity switching section, and a control device. A degree of the accelerator opening relative to the operating amount of the control lever is switched by the sensitivity switching section operated by a boat operator. The sensitivity switching section outputs the degree of the accelerator opening relative to the input operating amount of the control lever as a sensitivity switching signal. The control device controls output of the power source based on the operating amount of the control lever and the sensitivity switching signal.