Back when i was teaching around the Yucatan, one of the final things I used to cover with students (extra emphasis for the Euro divers!), was the importance of tipping the boat crews.
I would bring up their usual objections, (that "tipping wasn't customary back in Germany", or where-ever), then I would point out that this isn't Germany (where the same $55 dive would've cost them $200!), that this is the 3rd world, and these boat crews are trying to support families on very little money, and tipping is customary in much of the world, and in much of the dive business, so they're expected to follow suit, (unless, of course, the crew somehow screws the pooch and gives them good reason not to). . . .
I’m not sure I follow the logic. Tipping as we know it today was not customary in the “3rd world” or anywhere else outside of the USA until Americans introduced the custom.
Also, I believe the tipping for the purpose of helping support families conflates the original purpose of tipping—to reward service that’s above and beyond what their wages are supposed to be for—with charity. If I want to help the local people who have a difficult time getting by, I give to a charity or try to do something along those lines. If I want to reward a boat crew whose efforts to make my trip memorable went above and beyond what was expected of them by their employer, I tip them.