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).
These crews generally work their butts off, quickly switching your gear, bringing you drinks, fixing your malfunctioning reg or busted mask strap, dropping extra weights in your BC pocket (because your tubby ass insisted you only needed 12 pounds!), and generally catering to all sorts of picky,unreasonable expectations, etc.etc., and they do it all, in the hope of getting
tipped........so don't be a dick and stiff them !!!!!
Another thing i would point out, is to NOT just hand money to one of the crew, and assume he was going to be fair and split it with the other guy(s). If you hand money to the DM or captain, get the attention of the other guy, so he sees the tip taking place, and make it clear that it's for everyone. I've seen the first guy to get his hands on the tip, stash it quick, and never mention it to anyone else (in fact, that was the norm rather than the exception).
It's also nice if you tip openly and audibly in front of the other divers, which puts them on the spot, encouraging them to tip, as well !!!!
Your boat crew will absolutely love you for it, and they'll remember it !