An apparatus that attaches to a moving, typically man-made, underwater object and that is capable of sending a signal is disclosed. The apparatus includes a housing, which in some embodiments is configured to look like an underwater organism. Within or on the housing is a coupling device by which the housing couples, advantageously reversibly, to the underwater object. The housing also incorporates a transmitter that generates and transmits a signal. The housing also includes an energy storage device (e.g., capacitor, battery, etc.), which is used to power the transmitter and, as required, the coupling device. The housing also incorporates a generator, which generates the energy that is stored in the energy storage device.