This is maybe just me but this tipping thing is getting crazy. They chose that field of work just like you choose your field of work. Ask yourself do you tip your college professor, school teacher, guy that changes your oil, grocery bagger, bus driver,sales person at the mall?
Probably the salary they get, which is not comparable to any of the professions you've listed (yes, probably even the grocery bagger...).
Some jobs, in some locations, are tip reliant. That's a fact of life.
No and the thought probably never crosses your mind so what is so special about a Dive Instructor or a DM.
The potential variation in course quality, coupled with the intrinsic benefit of having a positive, high-energy instructor.... all factors which the instructor is not mandated to provide each day.
You've obviously not worked in the dive industry... so maybe you are under a mis-appreciation of the lifestyle.... but 'painting on a smile' and throttling up to max energy and focus every single day, dealing with 'difficult students' without exasperation, being empathetic always, applying the full weight of your experience to problem solve solutions to individual student training barriers... endless 12-18 hour days... no pension, no medical, no dental, no sick pay, no expenses paid to cover your equipment and membership costs.. full legal responsibility for student safety... 100% duty-of-care... upwards of $10,000 spent on gaining the appropriate training and qualifications to teach you....
Shop worker has that responsibility?
College professor has to apply risk assessments and un-wavering focus, hours each day, to ensure student safety?
Bus driver needs to empathize with student fears and difficulties...applying unique experience to solve a myriad of learning difficulties, stress manage and motivate their passengers?
Yeah.... I see how that correlates to a 'grocery bagger' indeed...
Or a driving instructor? How much is their hourly rate? Compared with the small % of course costs paid to an instructor for working how many hours, over how many days?
And yet, in some places, an instructor might earn less than $50 per student, per course (spread over 3-5 days).
So, it's wise to spend a moment observing what the instructor is giving you, over-and-above, plus considering what their % of your dive course costs they actually receive (often less than 30%) spread over the number of days on the course... where they live - cost-of-living there, and a bunch of other factors specific to your experience as a student.
Of course, if you want to value them like a grocery bagger, then do so.... but I wouldn't expect much in return... just your c-card 'bagged' and delivered..with minimum effort on their behalf.