I'd think some kind of (1-way only, obviously), comms system could involve things attached to the exterior that are deployed by bluetooth or something, and floa

I'd think some kind of (1-way only, obviously), comms system could involve things attached to the exterior that are deployed by bluetooth or something, and float to the surface regularly. e.g. every 2 hours, send up a transmitter that sends an "A-OK" on X Khz for at least Y minutes. If you get a bunch at once, or don't get any for a while, or get the "Something wrong" on Y Khz transmitter, you know something's gone awry.