A diagnostic system is provided for a watercraft that is propelled by an outboard drive. Sensors can detect conditions of components related to at least the operation of the outboard drive. The diagnostic system can leave inside and outside computers. The inside computer is placed in the watercraft, while the outside computer is placed out of the watercraft. The outside computer is installed with a program to provide the inside computer with an instruction signal to start a diagnosis on the components. The inside computer is installed with a program to provide sensors with a commencement signal to start checking the components, to require the sensors to provide a detection signal, to collect detection signals, and to provide the outside computer with a result of the checking based upon the detection signals. The program of the outside computer additionally controls a monitor of the outside computer to indicate the conditions of the components based upon the result of the checking. The inside computer can create a table listing checked components and provide the outside computer with the table. Alternatively, the outside computer can be eliminated and the inside computer can display the table.