hope there is a viable rescue operation near you.
I think this thought and a sense of realism is key !
In many of the locations in Asia, Maldives & Africa, the dive boats will have a marine radio. I have been on boats which have no compass or other navigation equipment, but the navigation skills of the crew were impressive. A DSMB and a horn, maybe a mirror, has to be the primary means of attracting attention in these locations. Unless you are in the first world or on a liveaboard with fancy navigation systems, PLBs and MRGs are wishful thinking. I can assure you that large commercial vessels which do have the fancy equipment are not interested and are not going to divert from their course …
I have been on sailing vessels crossing shipping channels trying to avoid collisions, and tried to contact large commercial vessels on marine radio, having identified them by name via AIS, and they just ignore you.
Good luck … the manufacturers of PLBs and MRGs want you to believe that vessels will respond. My own view is that Lifeboat organisations (if there is one), Coastguards (same), western operated leisure craft (liveaboards and yachties) in the vicinity are most likely to respond. These are thin on the sea outside the first world. Local craft don't have the fancy equipment. Large commercial craft are not interested.