This is based on the story of Tom and Eileen Lonergan who were left behind during a dive trip on the Great Barrier Reef in January 1998. The crew of their dive boat, Outer Edge, either miscounted or didn't count heads before returning to Port Douglas. Spoiler:
Tom's dive bag, with his wallet inside, remained on the boat for two days, and two other dive trips, before it was opened and someone raised the alarm. They were never found.
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I remember when this happened. It made a big splash in Australia. Dive tourists are the bread and butter of northen Queensland.
I was there at the time and had actually dived the same area of the reef earlier that month. I had chartered a different boat though. I remember replaying in my head how my crew had meticulously counted heads after each dive.
As one who has dived with sharks and other nasties in the ocean, being left behind was never really a primary concern. I never really thought about until then. I can't imagine the horror of surfacing only to find an empty ocean.
Edit: added spoiler...sorry.