Last year a dive boat left a couple behind in Key Largo Florida. They spent an uncomfortable and unhappy night on a small concrete light tower along the reef. But at least they survived. Also, when I lived in Colombia, a couple went out with a fisherman (there were dive charters available but they were trying to save money!!!) and got left. They were getting swept out of the islands to open ocean, but managed to ditch their tanks and swim to the last island before the open ocean. A search party found them, too. The fisherman had panicked and gone back to town to find help... The moral of those stories: make sure you dive with a reputable operator!