A boat propulsion system includes a drive bevel gear fixed on a lower end of a drive shaft, a forwarding bevel gear and a reversing bevel gear rotatably fitted on a propeller shaft and meshing together with the drive bevel gear, and a dog clutch mounted on the propeller shaft and rotatable with the propeller shaft, moves in an axial direction of the propeller shaft, and meshes together with the forwarding bevel gear or the reversing bevel gear to rotate the propeller shaft, in which the propeller shaft comes in contact with the forwarding bevel gear via a forwarding side buffer member having a disc spring as a component member. The boat propulsion system prevents and minimizes sound and impact even when variations in thrust force occur.