If they fall into the third category only, then I suggest that they consider the NAUI master diver course instead. That one was, for me, by far the most challenging of the courses I have completed. (Although foreign agencies, oddly, do not consider it that way. For example, in France I am considered "niveau trois" by CMAS only because of one of my TDI tech certs, "extended range", even though that course far less demanding, both in the diving skills and in the written examination, than the master diver. A search of the BSAC website yields the same result.)
Still, I consider the master diver course the most challenging course I have completed. I had already the Rescue Diver cert when I did that one, but I had to do another rescue portion for it. The course had nine dives total, even though the NAUI regulations only call for eight, because the instructor insisted on one planned decompression dive. I had a number of navigation exercises. Some with a compass and some with dead reckoning. I had to show that I could tie about six different knots, each within a few seconds. I had to learn nautical and boating terms and on-board etiquette. I had to complete the "stress test." I also had to retrieve a 25 kg Danforth anchor from ~80 fsw. I was given a six-foot piece of rope and had to tie one bowline to the anchor and one to the bag, bring it up to 40 feet, and keep it there for about 30 minutes in a current, dodging hills in the terrain. But I was allowed to attempt that only after I had demonstrated to my instructor's satisfaction, and with a pencil and paper and calculator, the relationship between the pressure and depth and amount of air in the bag at that depth, etc.
I made a 98% on that written exam, and it was the lowest score I have ever made on one of those diving exams. I'm a real study geek. I practiced every single practice problem in that manual and in every manual I have ever had for any cert. I already had a PhD in chemical physics by the time I started diving, and by the time I got the master diver cert I had worked as a post-doctoral fellow for the US Department of Energy for many years. For me, getting only a 98% on an exam tells me that it was difficult and challenging.
I'd suggest that if a person only wants the depth of knowledge of a DM, and the challenge of being put through the "stress test"--and the practical experience of being put through multiple navigation and rescue and recovery exercises, as well as leading dives, but who has no desire to actually make a living by being responsible for the lives of other divers--then that person might consider the NAUI master diver course instead.
But then, the instructor is what makes all the difference. Be sure to vet the instructor thoroughtly before hiring him. Or her. I paid $750 for that torture (nearly double the going rate at the time for that course). But the instructor came well recommended. I have since hired him for other training, including, trimix, advanced trimix, blending, and sidemount. I even asked him about DM. His immediate question was "Do you want to dive for a living?" When I answered "Hell no, I have a day job and it pays more. This is for fun" he immediately discouraged the DM course. "You don't need that," he said, "beyond what you learned in the master diver course, all there is in the DM course is some business/client skills, and you already know how to add and subtract and multiply." I may be misquoting slightly, as that was 17 years ago, but that was the gist of it.
