I know of no such app, although there are bluetooth devices that give that kind of alert.
I think you are making your app spec a bit overly complex.
I'd buy one in a minute, and all it would need is a bluetooth poll
of all the cellphones on the crew list... if one did not answer, sound a local alarm
, and note a GPS position. If each phone on board polled all the others on board, no matter who went overboard
, you'd know in a very short time.
The external call isn't much help to me since I mostly sail far as far away from cell phone signals as possible!