The less people in the water,the better for all concerned. Safety is something I do think about before starting the shoot. On Time Trax up the Great Barrier Reef, I had up to 12 stunt people and actors in the water - the open ocean. This has to be coordinated topside, where each person has to go, what they have to do, control the light scenes and try not to have endless retakes or people heading to the surface to discuss what occurs next. Underwater storyboards help greatly in this matter if underwater communications can't be used.
Safety is very important on underwater locations.
A complete checkout of both the diving and camera equipment is carried out by the divemaster and the cameraman. Underwater, the divemaster makes sure everyone follows the dive plan and provides back-up assistance to the cameraman or any other person, should an emergency situation develop. If there is a large underwater crew, more than one safety diver would be required. I can usually tell what can and cannot be done. As far as filming and diving is concerned, people who are not expert divers should not be filming.