I always factor in 10% for tips before signing up for a liveaboard or even land-based dive trip. Depending on the quality of service I receive, the tip could go up or down a bit. I have a very good idea of how much salary the crew, dive masters, instructors, kitchen staff are paid. It is not much compared to all the hard work they put in just to keep their guests happy and worry free. I have observed dive staff wear many hats day in and day out on a liveaboard trip. They work non-stop. While observing the crew, I try to imagine myself doing their work so I can appreciate their efforts better. This way, when I tip, I tip with a gracious heart.
I would like to comment on what 'Seya' posted above about 'just paying their employees a fair wage.' The dive industry is quite mature and very competitive. It has established a pay standard that is expected by someone looking at making a career in the dive industry. I am sure a dive master or instructor won't work for a particular dive op if the pay is below industry standard. It is like someone accepting a job and then grumbling about their pay later. If you do not want the pay scheme or pay rate and benefits being offered by a prospective employer then don't take the job. I don't know the actual pay rate of dive op employees but I do know that they are paid a base salary plus commissions just like a salesman or a restaurant waiter or a barkeep. The best thing about this kind of pay scheme is that the employees are motivated to work hard and give their all to earn extra money through commissions which is not a bad thing. You earn what you sow. You do good, you earn big bucks and a good reputation. The bad thing is when you pay someone a large salary every month and then that employee provides you with crappy service simply because they simply do not care. There is no motivation for them to do so and aim high. Employees like these are mostly found in most government offices and most unionized work environments. They leave a very bad taste in my mouth every time I encounter them in my day to day professional life.
As for the 'tip speech' given by some liveaboard ops, I do not take it personally. I see it as just a blanket statement of sorts. Remember that these businesses cater to all kinds of divers from all walks of life and cultures. Some people do not know how to tip and why they should tip. It is just that simple. I believe that the crew is just helping their customers realize the value of tipping. At times, they have to resort to acting or sounding blunt to get their message across to dense customers. While I do not support this kind of method, I do believe in employing subtle hints and leaving it at that. If some customers do not tip then that's how it is. Just move on.