I have worked on boats for many years and in 5 different continents. Generally I find that tips are a personal thing based on service received and that is what it should be. If the instructor, guide, crew do a good job and you get a good service, like in a restaurant you give a tip. It is not expected, but always gladly received.
Having read through some of the comments I am surprised on how much many of you think we earn..... it really isn't as much as you think.
I have worked for 30$ a day, and this is an 11 hour day, hauling tanks, filling them, washing gear. I have regularly done 6 training dives a day just to fit in what needs to be done, but with no extra pay. We do this job because we love to do it, yes, but it is also hard work, if it doesn't look like it to you when you are on the boat, then we are doing the job correctly as you don't see what is happening behind the scenes.
The cost of the course or trip is not reflected in the pay the DM or instructor receives in most cases, often if we have 4 students we get the same pay as if we had 1.