I have an ENTIRELY different take on diving than you and many others.... I see the ocean as a great wilderness, and exploring it as a sport.
I am fine with mentoring those that I think could be good at diving....and I do not believe that everyone should dive. I don't teach for money....never did, never will..... So this attitude of some instructors that "the right instructor" can teach anyone, is not something I care to endorse.....In fact, I think at least 10 to 20% of people that are going out on charter boats to dive off the keys, Cayman, Cozumel, or Florida, should never have been certified--should have been failed from the class.
When a person is afraid of the water, they don't learn well. When they are phobic about it, they don't learn at all. People with these issues come to diving, and try to prove to someone that they can beat their fears--but this is a foolish gambit that might work once in a million times. The instructor that actually cares about people, rather than just caring about expanding the market base of scuba..this instructor will HELP the phobic by explaining to them much better options for them exist, than their becoming a scuba diver--or even a snorkeler for those that get panicked with a snorkel.
So you as a tennis pro, can see a gifted athlete you immediately know should be coached to compete.....you can see the average player that should get weekly lessons for a very long time, and they can learn to become a B player or so.....and be quite profitable for you and the complex/Country Club, whatever...and you can see those with no hand & eye coordination, who will never be able to play well ---but who with the right drills, and the right instruction, can learn to ENJOY the game..even if they can't ever be great at it.... Unlike diving, their lack of any real ability will NEVER KILL THEM.