The most likely explanation here is that they never got on the dive roster in the first place.
I can’t tell you the number of times I have seen divers show up late to a busy dive boat getting loaded up, crew running around taking care of last minute diver problems... "I can’t find my fins!" or whatever… 2 divers get on board and chat through the dive brief and the first role call and no one notices that they never even got on the list.
Before you spend all your hard earned $ on new gadgets and GPS systems how about SHUT UP during the role call and dive brief and MAKE SURE your happy a$$ is on the roster? This is an easy way to help save you own life.
*Side Note* all the stories about this in the media have been wild and ridiculous, “Divers Left to Die at Sea” very sensational and play on everyone's fears. However, the biggest problem these guys had was NOT getting hit by another boat while there were in the water. They were smack in the middle of one of the busiest recreational boating areas on the planet and within sight of land the entire time. I dive here every weekend and can’t imagine how they went 2 hours and how many boats they allowed to pass before they decided to flag one down?
If you come to South Florida to dive, sure enough bring your SMB, you will need to wave at knuckleheads trying run you over, or maybe at pretty girls on the beach, but not because you are “lost at sea” less than 2000 yards offshore.