It is the Dive Boat Captain's job (or person in charge of the diving) to count the number of people on board before the dive and make sure that everyone is back on board before leaving the dive site.
If they do not do this then they are at fault and irresponsible, licence to carry divers should be revoked etc etc.
IMHO, if they ONLY do a count and do not do a face recognition check in, there is an increased chance they will count inaccurately and miss a diver. This has happened in several incidents.