I have been on both sides of the always-impassioned tipping debates here, and not to play devil's advocate. I genuinely enjoy tipping when I feel it is appreciated, just as boat crew probably enjoy their jobs when they feel appreciated. The problem arises when people campaign for (or against) tipping, adding an extra burden of obligation and, perhaps, resentment. Nobody likes being told that they
have to do something.
If the bi-monthly tipping discussion confined itself to what is customary, it might be more informative and less likely to get bogged down in the usual arguments and passions.
For my part, I have been tipping $5 a tank in Bali lately, and $2 to tank porters. That seems to be in excess of the norm. I am happy to tip the same for solo dives as well, since I want everybody's interests aligned, as we say on Wall Street.
I tip 10-15% on liveaboards, though it's been a while, and that seems to be in excess of the norm as well.