I think this thread exemplifies the differing attitudes between North America and the rest of the world towards tipping.
The only other point I would make is that scuba diving instructors in resorts have made that personal choice " to swap the ocean for the office and live the dream" as PADI call it. They usually accept that at best they will eke out a day to day living out of diving and get a roof over their heads and beer in hand at the end of the day .They view healthcare, pensions and owning a house as "materialistic" western things. This is their personal choice. If resort instruction is an overcrowded field and people struggle to make a living so be it. PADI certainly won't give a fig- their only interest is to have courses sold and rake in affiliation fees.
Remember that you are unlikely to see these resort instructors again. They are your friend while there but won't be interested when you go home. Remember also that as part of the PADI IDC they have been taught to sell courses in role play situations with the Course Director advising while one wannabee instructor plays the instructor selling courses to tourists while the others watch. They then swap around. Would you remotely contemplate tipping the car salesman or double glazing salesman? No of course not. By all means buy the resort instructor a beer at the end of the day but tip no. They'll soon forget you and move on.