You're comparing something like a university professor to a scuba instructor? Scuba diving is a super easy to learn hobby that hardy requires any skill to teach. A professor has gone through years and years of full time work, study and competition and teaches on a way higher level. You're getting paid pretty good money for this, at least in Europe.
That's truly wrong. Please note that I am both a diving instructor and a full professor at the university.
I did take me 3 years for qualifying as a diving instructor, after my first OW course (which was another year).
There were no shortcuts, the regulations required first to become a full certified 3-stars diver, then to serve as an assistant instructor for two years, abd finally I was admitted to the instructor training course, which was very demanding both physically and mentally, and required to show deep knowledge of physics, physiology, psychology, environment, marine biology, deco theory and practice, boat handling, rowing, doing knots, free diving, using CC rebreathers, resuscitation and salvage, etc.
On the other hand becoming a professor at the university was much simpker.
I had to file a form, attaching the best 10 of my 300+ scientific papers, and passing a 30-minutes oral interview, including 10 minutes of simulated lesson on a given topic and a 5 minutes reading and translating test from English to Italian.
Also during lectures, teaching diving is much more demanding, due to physical effort, the management of adverse environmental conditions and the responsibility on life and health of students.
All this said, I entirely agree that it is a bad idea to tip a teacher who has the power to pass or fail his students.
It is truly unethical, and potentially very dangerous.
As an university professor, I would seriously risk my job if accepting even a small tip or gift from a student.
And if I am caught soliciting a tip, I would be arrested and jailed for a couple of years.
A diving instructor probably does not risk such extreme consequences, but the ethical misconduit would be the same.