What is the purpose of tipping in this instance? A tubbataha liveaboard is expensive as it is, would the staff not make an adequate wage to begin with? Or would they just increase the prices by 5% if they needed to pay their guides more?
I can understand at the small eatery you stumbled into, or the small shop where there isn't much business or its the off season, but peak season in a well sought after location with a hefty expense already seems like service should be included?
I travel quite often to the US, and familiar with the controversial "expected tipping" behavior in restaurants, deliveries etc.
But in this instance- from my experience there is a discrepancy between the cost of the liveaboard (which *is* expensive) and the local crew salaries. The kitchen staff, cleaners, compressor, dingy drivers- sometimes the guides- in many liveaboard operations they don't get a salary that remotely reflects the price that we pay. Sure, I understand that in a utopic world they should get paid decent salaries, maybe in par with the liveaboard costs. In reality, most of the sum we pay goes to owner- profits, boat maintenance, whatever.
The tip is to make some difference. Sure, we can ignore and say, "not our problem: the owner should pay them good salary". But we are not going to fix the world, at least not in liveaboards in developing countries. The crew works hard, and I think the tip does help them a lot. And because as you mentioned, these trips are awfully expensive I, for one, would like to tip something decent: not 20% which is a huge amount of money, but also something that is adequate and will be appreciated. There are chances I won't meet the same crew again, so I expect nothing in return - but on the same day a new group of guests' is coming on board, the hard-working crew will at least start a new week of hard work with a smile.