An engine for a personal watercraft uses an open-looped cooling system and comprises a cylinder block having a water jacket within which cooling water flows, and a piston that reciprocates within the cylinder block, and a dimension of the water jacket in a reciprocation direction of the piston is equal to or less than a half of a reciprocation distance of the piston. An optical filter is structured such that an upper limit of a transmitting wavelength decreases as a distance from an optical axis of the imaging optical system increases on an imaging plane of the imaging device so that a light in an infrared region is transmitted on the optical axis and cut off at a position farthest away from the optical axis on the imaging plane. A selector mechanism for selecting at least between pivotal and rotational movement of the plunger, wherein the pivotal movement relates to orbital blade motion, and the rotational movement relates to scrolling blade motion. Accordingly, in one embodiment, the receiver uses spatial averaging to average out this oscillating error.