Abstract: The invention is directed to a method and an anti-theft device for a personal watercraft which is powered by a gasoline engine-driven jet pump. The anti-theft device, having a nozzle cover, a cover holding mender and a cover locking member, is designed to be sealably attached to a jet pump steering diverter nozzle in order to prevent and deter theft of such a watercraft. The jet pump steering diverter nozzle, having an entrance and an exit end, is locked by mounting the nozzle cover to the exit end of the jet steering diverter nozzle, the nozzle cover is engaged in tight contact with the jet steering diverter nozzle with the cover holding member, whereby preventing movement of the nozzle cover in order to restrict the flow of water passing through the jet steering diverter nozzle, and the nozzle cover is locked to the jet steering diverted nozzle in order to prevent unauthorized removal thereof.