The maritime equivalent would be the combined EPIRB+voyage data recorder. The engineering would need to be substantially different as the worst they will need to survive is something like a 35m drop test into water.
In terms of energy budgets, any device that is not related to distress alerting ie the FDR/CVR would be better as a transponder only. There is no point in transmitting 'blind', wait until you know someone is actively looking before using any energy reserve.
RF transponder tags like RECCO are 'powered' by the received RF energy and are only effective over very short distances (metres) at the frequencies used, this would be cm in water.
Last edited by ionagh; 12th Nov 2009 at 13:56.
Reason: Recco notes