The only problem I see with "built in" tipping is the staff knows they will get a bunus regardless of the quality of service they provide, so after a while they may have a tendancy to not work for it. It diminishes the incentive.
It seems logical, yet I am not completely convinced that service industry people tend to work harder when they know the tip is not a fixed amount. It seems to me that after a while they might come to the conclusion that some customers are generous tippers and others stingy or just difficult to please. People rarely adjust their tip THAT much based on service; I mean, a difficult customer might tip "only" 10% in order to "teach the employee a lesson," and a person who thought the crew went the extra mile might tip 25%. So, on average, the person probably ends up taking home the same amount in tips regardless of whether they go the extra mile or not. Also, I think service industry people who work hard often do it more out of a sense of pride in their job than anything else. Their managers know whether they do a good job or not, and whether customers like them or not. A truly surly crew member won't last long. Lastly, in cases in which tips are pooled, which I believe is common on liveaboards, each crew member gets an equal share, and it probably works out to the same amount each would have gotten had the liveaboard simply added a 10% or 15% gratuity to each diver's bill.