Tipping is "supposed" to be for special service. But in reality, workers in industries were tipping is expected get paid a pittance, in the expectation that they'll make money on tips. In theory, the good workers will make good money and the lousy workers will not. But there are so many other factors. E.g., a waiter in a restaurant where meals cost $100 makes a LOT more on tips than waiters in restaurants that charge $5 for a meal. Some people tip generously, while others say, "I'm paying X for the trip, why should I have to tip?"
In a fair world, everyone would get a fair living wage, and tipping really would be for service above and beyond. But since we don't live in that world, I try to find out what's considered a "proper, decent" tip and then I add 50% to it for regular service, or 100% for special service.
When we first started diving as a couple (a little over ten years ago), we tipped $10 per person for a two tank dive trip. A couple of years ago, we figured with global inflation that wasn't enough, so we increased it to $15. Also, we try to be sensitive to exchange rates, so if we are tipping in the local currency, we try to make sure that it still works out to $15 per person per tank.
Finally! Someone who specifically acknowledges that the "$5 a tank is the going rate" advice that's be touted for here for going on twenty years is at least ten years out of date. $15 per per person/per tank is on the very generous side of the current going rate of $10/tank.
I'm a little unsure whether ibj40 is tipping $15 per two-tank trip, as the second sentence seems to say, or $15 per tank, as the third sentence says.
I think I'll adopt as my standard $15 per tank for normal service and $20 for first-rate service.
I've never received less than good service, and usually I get first-rate service. Maybe it's the sort of places I go. Once, and only once, I had a DM who did the basic expected work and no more. All the others have been people who seemed to love what they were doing, and love helping people, and were always happy to do whatever it took, like lifting my tank out of the water on account of my bad back.