The biggest danger divers have of being left behind is by not interacting with other divers on their boat. My friend was captain of the dive boat in Key Largo, Florida who left a couple out on Molasses Reef a couple of years ago.
They climbed on a buoy and stayed the night. My friend had his captain's license suspended and decided to get out of the business. The large hotel chain that he worked for had a scheduling problem and at the last minute called a young man who was a very new dive master. He made the mistake of counting heads instead of checking people off of a list. Divers, like cats, tend to move around and the captain is ultimately responsible for anything that happens on the boat.
This couple had kept to themselves so there was no one concerned about them. I tell all new divers and especially those who are worried about going to the Blue Hole to make sure they CONNECT with someone else that will make sure they are back on the boat.
The original question was about drowning. There are a number of things that could lead to drowning while diving. One is being over weighted, getting down to far and not being able to compensate with air in the BC. This probably would only happen if someone is too tired to kick up, totally oblivious to the fact that they are descending or one of many complications of nitrogen narcosis.
