Believe it or not, but it is not uncommon for dive operators to pull away from a dive site before they realize they left a diver behind.
As horrible and irresponsible it sounds, it does happen. A dive operator leaves a dive site only to discover that a diver is missing. It is quoted in the following blog post, cited below, that one dive operator actually counted fins on the dive deck to determine if everyone was back.
Moral of the story: Some dive boats, even in First World countries, still don't have foolproof systems to count divers before departing dive sites. Some boats do have good systems, either voluntarily or because the country they operate in requires it. But there are boats, from Key Largo to Komodo, that can be lax, even downright sloppy, in ensuring everyone is back aboard. That's why you must find out what their diver-counting system is before you jump off the boat.
As simple as it seems, the lack of a reliable basic counting system is the root of the problem. Tagging systems such as the Dan Diver I.D System as well as the ScubaDiverTag Diver Down system provides convenient and easy way to track divers that are in the water.
Open Water filmmakers Chris Kentis and Laura Lau say the inspiration for their low-budget movie, shot during the couple's weekends and vacations, was the real-life story of Thomas and Eileen Lonergan. The young Americans, who never were found, were stranded in 1998 when their 26-passenger vessel departed without them from a popular dive site along Australia's Great Barrier Reef. The incident prompted a tightening of the Queensland dive industry's already stringent safety rules.