The following is a piece which appeared in edited form in my "Ask RSD" column in Rodale's Scuba Diving
"How much, when and whom do I tip on a day dive boat?
As with tipping in other situations, dive boat gratuities are a personal matter, although local customs should be respected. For example, tipping is not yet customary in Australia, New Zealand, Japan and other countries.
In many locations, including the US, Mexico and greater Caribbean, however, it does seem generally agreed that tipping for good service on a dive boat is appropriate. Keep in mind that some crews derive much, and occasionally nearly all, of their income from gratuities, regardless of whose “fault” this is. Repeat customers of a dive op can well vouch for the value of recognition of crew effort.
For those interested in tipping, guidelines for day dive boats include: (1) Gratuities are just that, something given voluntarily and beyond obligation for a service rendered. They are not mandatory; (2) An appropriate amount is somewhere between 10-15 percent of the cost of the diving expenses, not including gear rental, park or chamber fees, and other incidentals. This amount should be adjusted so as to fit the level of service, with a range of 0 to 15 percent being acceptable, although some generous divers do leave more; (3) In most situations, you need tip only one crew member, often the DM who primarily attended you. The tip will be shared among the crew according to their internal agreements. If a particular crew member rendered a special service which was above and beyond the call of duty, e.g., brought a piece of his personal gear from home for your use, cleaned up after you following an episode of sea sickness, etc., then an individual gratuity could be justified; and (4) If your crew changes every day, tips will probably be most fairly shared if given daily. If your crew is stable and you desire to consider tips at the end of the trip, let the DM know. Of course, human nature being what it is, the immediate rewards of a tip at the end of each diving day may result in the best service.
Finally, if in doubt about tipping, ask around. Local divers can tell you about their customs, and the consensus of your fellow divers will give you a good idea of what is appropriate."
You also may wish to read the very recent and informative Undercurrent
series on tipping, Tipping on Dive Trips: Part I
(May, '08), Tipping on Dive Trips: Part II
(June, '08) and The Last Word on Dive Tipping
Hope this gets you started.