A theft deterrent apparatus in a marine vessel having a propulsion device includes a key unit arranged to transmit a user authentication code, a first authentication unit disposed apart from the propulsion device, a second authentication unit, and an operation control unit disposed in the propulsion device. The first authentication unit is arranged to receive the user authentication code transmitted by the key unit, execute an authentication process on the user authentication code, and generate a unit authentication code. The second authentication unit is arranged to receive the unit authentication code generated by the first authentication unit and execute an authentication process on the unit authentication code. The operation control unit is arranged to allow operation of the propulsion device if authentication by the second authentication unit does succeed, and prohibit operation of the propulsion device if the authentication by the second authentication unit does not succeed.