If you want to tip for people helping you on with your gear, some stale, dumb jokes and the calypso dance, fine. Just don't tip because you THOUGHT they kept you safe, or had ANY responsibility for your safety.
I don’t wish to rattle the cage here because I am primarily in agreement with the sage advice being given by lots of people. Every diver should be their own guardian angel and never become complacent by assuming that a DM, a guide, or whoever is going to ‘keep you safe’. I can put my own gear on thank you, and I will take care of my own equipment for that matter.
At the same time, tips don’t have to be tied to dancing, stale jokes, and the rest. They can be tied to safety just as easily if DMs and boat crews exhibit the appropriate behavior one expects.
If you have a DM who does excellent dive plans that turn out to be spot on dive after dive, if the DM makes wise decisions in the face of current or other unexpected contingencies and shows that he or she knows the area and the conditions so that you don’t end up getting unduly tired or what have you, if the DM handles other guests who act recklessly by speaking up and saying something instead of kowtowing for the tip later on, if you have checked the air and it is safe each time, then by all means give that DM and the supporting crew that tip when all is said and done and make sure there is no ambiguity about why they are getting it.
I tell the DM and the crew as well that the tip was tied to excellent dive plans, crews that were alert and helpful, or that those mentioned plans were followed when we hit the water. I tip and I fill out the comment card, and I avoid unnecessary platitudes about how gregarious the DM was, how fantastic the food was, how comfy the rooms were, etc except perhaps as a footnote after pointing out what is really important to me.
The tips themselves aren’t the problem, I’d say. A lot of DM’s and crews need them to make a fair living. Just make sure that the tips you give are tied to specific behavior that leads to safety and you can be sure the behavior will be repeated for others who are hopefully likeminded. And if you don’t like the service for safety reasons, withhold the tip altogether and tell them flat out why. That should be as much a consequence in my opinion. It isn’t like at a restaurant in the US where even the crummiest waiter still pulls 10% if the service sucks. It is an all or nothing deal for me.
Cheers!