A watercraft positional information managing system comprising: a server; and a watercraft including a propulsion device and a communication device configured to perform communication with the server, wherein the communication device includes a position detecting section configured to detect positional information of the watercraft, the propulsion device includes an engine and a controller configured to control the engine, the controller being connected to the communication device, the controller is configured to output data related to the engine to the communication device, the communication device is configured to transmit the data related to the engine inputted thereto and the positional information of the watercraft to the server, and the server is configured to receive the data related to the engine and the positional information of the watercraft, the server being configured to determine whether or not the positional information of the watercraft is genuine, characterized in that the determination whether or not the positional information of the watercraft is genuine is based on the data related to the engine, wherein the data related to the engine includes at least one of an operating time of the engine, a fuel flow rate, a speed of rotation of the engine, and a throttle opening degree, the controller is configured to encrypt the data related to the engine with an encryption key related to information unique to the engine, the controller being configured to output the encrypted data related to the engine and the information unique to the engine to the communication device, the communication device is configured to transmit the positional information of the watercraft, the encrypted data related to the engine, and the information unique to the engine to the server, and the server is configured to receive the positional information of the watercraft, the encrypted data related to the engine, and the information unique to the engine, the server being configured to decrypt the encrypted data related to the engine with the encryption key related to the information unique to the engine. a server; and a watercraft including a propulsion device and a communication device configured to perform communication with the server, wherein the communication device includes a position detecting section configured to detect positional information of the watercraft, the propulsion device includes an engine and a controller configured to control the engine, the controller being connected to the communication device, the controller is configured to output data related to the engine to the communication device, the communication device is configured to transmit the data related to the engine inputted thereto and the positional information of the watercraft to the server, and the server is configured to receive the data related to the engine and the positional information of the watercraft, the server being configured to determine whether or not the positional information of the watercraft is genuine, characterized in that the determination whether or not the positional information of the watercraft is genuine is based on the data related to the engine, wherein the data related to the engine includes at least one of an operating time of the engine, a fuel flow rate, a speed of rotation of the engine, and a throttle opening degree, the controller is configured to encrypt the data related to the engine with an encryption key related to information unique to the engine, the controller being configured to output the encrypted data related to the engine and the information unique to the engine to the communication device, the communication device is configured to transmit the positional information of the watercraft, the encrypted data related to the engine, and the information unique to the engine to the server, and the server is configured to receive the positional information of the watercraft, the encrypted data related to the engine, and the information unique to the engine, the server being configured to decrypt the encrypted data related to the engine with the encryption key related to the information unique to the engine.