I don't think the CG wants the "diver" to get and return the wristband. I think the the DM could place a tag on the tank valve just before the diver enters the water and removes it when the diver exits. This puts the DM in direct contact with each diver at the beginning and end of each dive. No matter what system you come up with, the human factor is the weak point. The Captain must be the the one to enforce the proceedure. If he doesn't hold the DM responsible then he is at fault.