I've been tipping $5 a tank, as many other things I had read made me think that was more standard. Seems like some people online, especially from the U.S. (which I am also), just love to tip more. I don't mind tipping more (and I have) for really great service, but I don't really go for the noblesse oblige view of tipping that some seem to have in every scenario. Please note, the prior sentence is not aimed at anyone in particular, just a general observation.
I will probably get backlash over this but I will be honest. I have been in situations where I did not tip. One such example...It was essentially a water taxi. No one helped me with any of my gear or tanks outside of just filling them for the next dive. I was throwing my fins up onto the dive deck right before I climbed up the ladder and grabbed them myself upon getting up. No snacks provided, just water. No help grabbing camera gear or getting in and out of my gear. No divemaster (which is what the OP is asking about)....Nothing. The crew would just chat between dives and do their own thing with each other; they didn't really chat with any of the divers on the boat. Yes, they were doing their job, but I really didn't feel like it was deserving of a tip. Did I require help? No, but it would have been appreciated. If that makes me a cheap arse and a jerk, so be it. I didn't believe in tipping just because that was what the expectation is/was.
On the other hand, I have tipped generously when the crew makes an effort.
I was also on a British day dive boat in Egypt and it is always interesting to see different tipping cultures. There was not one mention of tipping or a tip box at all for the entire day, nor was there a tip box to be found anywhere on the boat. The same thing happened on a day boat in New Zealand.
I give it a lot of thought especially on liveaboards because a lot of the crew sleep late and wake really early to make my trip as great as they always are. They are very accommodating to requests and go out of their way to make things happen with a smile on their face which is not easy to do considering the amount of time they are away at sea at a time and away from their friends/families who are often times not at the port they sail out of. It is my dive op, hotel, concierge, and restaurant all in one...7-14 days at a time.
As mentioned before, it's important to consider location and circumstances. In the Philippines, you can get guided boat dives for $20-30 a tank with nitrox and sometimes even including equipment. While one
could tip 20%, and it would probably be very appreciated, it isn't necessary when you take into consideration the local standard and customs. $10 a tank would be a gigantic tip. In order to determine what is appropriate for the OP, the question to the OP is.....where are you diving, what are the circumstances of the diving, and how much is it per tank? Is DM and crew mixed together, is the DM separate and the crew together, or....?