Sitting here with a keyboard in my hand it is really easy to come up with a plan that would benefit all involved... I am a fairly big guy, lift weights several times a week and am a semi-pro drummer - I would put my upper body strength against many. Here is my nice warm office evaluation <grin>:
Of course, the instructor was in charge. But, if a large person was about to drown a small instructor, I would probably have yelled out to the instructor to see what she wanted me to do. If he/she was having difficulty, I would have offered to throw the paniced diver something, make sure my buddy STAYED PUT IN THE BOAT NO MATTER WHAT HAPPENED, and followed the instructors lead. Being a DMC, I have been ResQ trained and had a similar incident happen in Grand Turk, incidently very soon after my ResQ class. The DM of the boat was the smallest guy I've ever seen in my life- very competent and capable, just very small. The paniced diver in this case was a HUGE guy, overweight a bit, I'm guessing 260+lbs. That poor guy just could NOT get a handle on the situation. We threw the big guy a life ring but he didn't even take notice of it. There were 3 others on the boat, my GF was also ResQ trained. I yelled to the DM asking if he wanted my help. He was choking water at that point, and I told my GF to STAY ON THE BOAT and I jumped in. I had to wrangle the guy a bit, but got his inflator and his tank from behind and finally got him to relax. Only when I manhandled him to show him I had control did he slow down. The other option I was weighing was, as the ResQ book describes, to let him wear himself down. Fortunately, it all ended good and I got a dinner with drinks not only from the guy, but the DM who was most appreciative of the help.
That's my story and I'm sticking to it. I would NOT have jumped into the water if A) the DM wasn't in trouble, or B) another person, or in this case, a known ResQ trained diver was in the boat to assist.
Quite the experience right off the ResQ course, wouldn't you agree?