Put me in the anti tipping group.
I expect to pay for a service up front not have a hidden amount that I am expected to pay afterwards.
If you cannot make a living at what you are doing then you should look for another job. There are lots of complaints from dive instructors here about how hard it is to make a living as an instructor. You made a choice to enter an occupational field that was already overcrowded, had low pay or margins etc. You did it with the knowledge that it was a low pay business, you chose to give up the office or the factory to get to dive every day. Your routine is what many people save up all year to do for a few days. Most instructors are quite capable of getting a more traditional job and earning more money elsewhere so why should I pay more than market rate for a service just because some instructor wants to live on a beach and dive daily?
I will tip for exceptional service over and above what I paid for. If I take a course and need a significant about of extra attention from the instructor or they go far beyond what I paid for, I will make additional payment. That is not really a tip but payment for something over and above what I originally paid for.
If I only need something explained or demonstrated again, that is part of the variation of the course. If a little additional explanation requires additional payment perhaps the instructor should refund me money if I don't need the usually amount of explanation.
I am an engineer, I understand the physics of diving better than most instructors, should I have gotten a partial refund because all that part of my OW course was the instructor questioned me to see that I understood it?
Don't get me started on the practice of tipping food service personnel. They should get sufficient base wage to make a living and only get tips for truly exceptional service.
I will show my appreciation by posting reviews, I post lots of reviews both good and bad.
I worked as a consultant and as a contractor for many years, no matter who I go for a beer with, I generally pay for the first (and often only round) to me that is just being friendly, I buy lots more drinks and meals than I consume.