Be memorable so stuff like this doesn't happen to you:
The Detroit Free Press:
Australian safety officials say they’re investigating a dive boat company that accidentally left a U.S. tourist behind while he was snorkeling on the Great Barrier Reef.The Charter Boat employee failed not only to have all returning snorklers sign in but also failed to notice a snorkler was missing.
Ian Cole of Michigan told The Cairns Post he panicked when he realized the boat had left without him on Saturday. He managed to swim to a nearby boat and was not hurt.
The best head-count systems in the world don't work if they're not used.
Whenever you get on a charter, introduce yourself to the captain, chat with the divemaster or other crew and make a lasting and positive impression so they want to stick around and pick you up after the dive or other activity is over.