Tipping isn't just a city in China.
You may have heard things like "Its their job, why should I tip them for doing it" or from our European friends "Tipping is an American thing, employers should pay them more." While there are merits to both these statements, they do not reflect reality. Your dive guide, Divemaster (DM) or instructor does way beyond what their job requires.
For example, did you know when you go out on a pleasure dive it's not your Dive professionals job to assemble your gear? They also have no obligation to carry it to the boat or even check if it works. You are a certified diver, that's on you. Think of it like a taxi driver taking you to the airport. Loading and unloading your bags is not an included service with your fare. They are going above and beyond. Your dive professional who literally has your life in their hands does the same thing, surely that's worth something right?
There is no written rule for tipping your dive professional so use what you know, service industry tipping starts at 15-20%. Of course you can adjust for quality of service. For a 2 tank dive costing 100 dollars, 20 dollars is a good tip. If you were "that guy/gal" on the dive, consider tipping more. Buying the dive staff dinner or drinks is great, most will eagerly accept, but that doesn't pay their bills so cash is always preferred.
Find out how tips are dispersed beforehand. Does the boat crew share tips with DMs and instructors or the other way around? Who do you give the tip to, is there a jar? These are questions you would ask the shop manager or local divers. Also don't be afraid to tip someone extra separately if they did an exceptional job.
Last thing. So you had a great time and tipped well, fantastic. You tell the shop you will leave a good trip advisor review and drop some names, even better. But what really helps your dive professional is to drop an email to their dive training agency and tell them of your experience. Be sure to include your dive professionals id number. For padi that contact is QM@padi.com.
So now you have an idea of how to tip and what deserves a tip, go out, Dive and make your dive professional smile!