What number of divers onboard makes determining their presence especially difficult? I don't know. Maybe 2.
I do know that when the number is up there, (24 in my experience) HEAD COUNT'S DON'T work! Probably as reliable as doing nothing.
No system is infallible yet some type of tag system or sign in/out sheet would be more reliable. Operators should do both when the number of drivers is up there. The key is to use the system.
I've never observed a head count taking place on any boat with any number of divers aboard. I guess I could have just missed it but its been a lot of boats. I've never been on a boat that conducted a roll call. Do you need to do a roll call on a six pack? Probably not. However, if they count heads, you're trusting they don't confuse the number of divers on board that afternoon with the number of divers on their morning boat. Some of the liveaboards I've been on use a sign in/out board or sheet but it doesn't take over a couple of days to notice no ones looking at it.
It's like, dive at your own risk. If we leave you behind rest assured we didn't mean to and we're sorry for the inconvience.
There's got to be a more reliable way and if it takes an insurance hike to make it standard proceedure, then so be it.
Working together, Dive Operators could design a standard proceedure that would be reliable. With a reliable proceedure the chance of a law suit is reduced.
I've never felt at risk of being left when on a six pack. When the count gets up to 10 or so I feel some degree of risk. If I was diving in Florida and the boat actually called roll, they would have me as a loyal customer for life.
P.S. All my diving has been Florida, Alabama and the Carribean, so for other places, I don't know what the typical procedures are.