My experience working as a Paramedic, Ambulance Station Chief and teaching/doing first aid for more years than I want to admit to changed my persepctive bit. The reality is as previously mentioned the best options are prevention and intervention
before the ship hits the sand!
How a person reacts to an emergency tends to be fairly consistent. People who are trained and/or inclined to STOP, THINK, REACT are likely to be the ones you can rely on. The person I will always note as potential help will be calmly going about setting up for the dive. They will be
listening to the dive brief and taking a good look around the boat, entry point and quietly scanning the other divers to figure them out. The person you can count on in an emergency is the one who can take charge but doesn't NEED to be seen as
in charge. In Oz we say they won't be "Big Note-ing themselves"
One of my favourite stories was told to me about a dive trip where a woman was going on and on about how lucky they were that she was on the boat. She was a surgical nurse and would take care of them all! This went on for 3 days before our friend got so fed up he said to her..."So you are a Surgical Nurse ... do you know what my wife does?" Obviously the answer was "No" His response was "She tells people like you what to do.. she is a surgeon" The rest of the trip was peaceful. Our friend the surgeon felt no need to carry on about what she could do.
Everyone hears about the Flight or Flight Response. We seldom hear about the Freeze Response which is just as natural... think Deer in the headlights (maybe if I am very still the danger will miss me and go away). Everyone wants to think they will always have the Fight response especially those of us trained to respond to emergencies. ANYONE is capable of a Freeze Response like the doctors
@Miyaru mentioned if the event is too far out of their comfort or experience zone. The recovery time for those who are trained is significantly reduced tho.
When I trained paramedics I always told them "If the difference between life and death is what you do in the next 10 seconds, the life you are most likely to save is your own. If you do not stop and think before you act there is an escalating risk of doing the wrong thing and making things worse."
Be aware, be prepared and always remember you don't live in a vacuum. Your decisions should always respect your commitment to being there to take care of your loved ones.