My attitude on tipping is, if I can't reasonably foresee a way the expectation of a tip improves the service, I don't do it. I tip my waiter, because he did a good job bringing me my food and keeping my drink full, and I want to encourage that behavior. I tip my barber, because she did a good job cutting my hair without being annoying about it (and I've got some horror stories there, believe me).
The extreme example of where I don't tip is a self-service frozen yogurt place. Your only job here is to keeping me from stealing stuff (and I guess refill empty toppings/churns, but like, realistically how often are you doing that on a Tuesday?). I see no way tipping improves this dynamic, unless I'm slipping you a fiver to ignore me walking out with $15 worth of froyo (joking, obviously).
So, for a dive instructor, my attitude would generally be that he's got enough incentives to do his job properly without needing a tip, since frankly, all I really care about an instructor doing is teaching me the skills in a safe fashion. I'm not looking to be shown where the cool animals are, or for help setting up my gear (except in OW, where that's part of the skill competencies). If he needs extra money to incentivize him to run a safe course, I shouldn't be here to begin with.
Bear in mind though, my one scuba class to date was a college course, 20 students/3 instructors/dive, so it definitely didn't feel like a tipping situation. I suspect if I was paying for the sort of private, one on one dive instruction that some people go in for, I'd feel differently, because at that point I presumably am looking for some extra attentiveness.
Similarly, when I went on a dive charter in Hawaii last year (only time I've done that), I tipped pretty generously. I was super new to diving, definitely the weak link as they say, and I needed help getting the rental gear set up (wasn't familiar with the model BCD they were using). Between that and it generally being a great set of dives, yeah, I tipped those guys well.
