I went to the Great Barrier reef years ago pre- Open Water, and I went on several types of snorkeling/diving trips. Some were small and everyone knew each other, but some were large boats with up to 30 people. Some people didn't even talk to each other, so if one or two would have been missing I don't think I would have noticed. I don't know if that was the case with this trip, but I've heard cruises on large ships do this type of excursion too, and if I were on one, I'd be shaking as many hands as I could and promising lots of people to buy then a beer afterward so they'd notice if I got left behind! Yikes!
Someone should make a diving mask with a flare built into it!
Worse comes to worse, you could just rocket out of there!