In many countries academic teachers, diving instructors make pretty good money. Instructor and guides, people waiting tables etc. don't.I had never ever bought anything for my academic teacher/lecturer, driving instructor etc etc.
Typically, it's low paying service jobs were people tip or receive tip. I'm happy to tip somebody when I feel they did a good job. You don't have to. I might not be a thing in Hong Kong but in other places it is. There is a cultural aspect to it too.
Some of you guys are hella stingy. Travel overseas to go fancy schmancy scuba diving and can't give a kid in a restaurant or the cab driver a couple of bucks. Really?