A method and device for processing self-diagnostic information relating to an operating state of a jet-propulsion personal watercraft to display the self-diagnostic information on a display device equipped in the personal watercraft. The method typically includes the steps of obtaining operating state information relating to the operating state of the watercraft, performing self-diagnosis of the operating state of the watercraft based on the obtained operating state information to obtain diagnostic data, determining whether or not an abnormality exists in the diagnostic data, determining whether or not an operation condition of an engine mounted in the watercraft meets a predetermined operation condition, the engine being configured to propel the watercraft, and outputting information of the abnormality to the display device based on a result obtained in the step of determining whether or not the abnormality exists in the diagnostic data and based on a result obtained in the step of determining whether or not the engine meets the predetermined operation condition.