It may be unlikely that all the things shown in the movie transpired, however, people have been left behind. In Australia, in Coz, in Florida and in California, etc. It happens. Each time it happens methods for counting divers supposedly improve.
A few months ago, I was slowly ascending the last few feet to the boat when I saw from below that the ladder on my boat was being pulled out of the water. Though I assumed it was being done as a joke, I still surfaced a bit faster upon seeing that.
I found out afterwards that the boat *had* been preparing to leave, but my friend said "Hey, you guys still have a diver in the water." I'd been diving with them for a couple of weeks already, so I'm reasonably confident they wouldn't have gone far before realizing I wasn't on the boat, even without my friend's comment. Still, you just never know. That's why I have a Dive Alert on my low pressure hose and carry a big dive sausage marker.