For me tipping is a matter of conscience and no, I am not Catholic. I do what I feel is right based on the type of trip, the individuals involved and the circumstances of the dive operation, country and/or area Im diving. Generally, the only reason I would not tip is if I got bad or negative service, or if I saw other divers being treated poorly. In those instances I have no problem not tipping.
If tipping guidelines are not posted the first thing I ask, before I even put a fin in the water, is "What is your tip policy? I like paying a flat $5 to $20 dollars per day versus a percentage, but only because I am mathematically challenged. If any crewmember, including the captain, goes above and beyond I will give them extra and make sure they know it is for them and them alone. I will not give individuals more at the expense of others who have worked equally hard. I know some dive operators frown on this practice when they have a crew kitty, because of "team work" and all, but sometimes I am just a bad girl and break the rules...no apologies.
As for a DM, I expect that if I personally need a DM for a one on one, I will pay extra for that need. For most operations, a DM should always be on board and calculated into the price of admission. I have also tipped my certification instructor(s)..they deserve it, and more, after working with me!
I may be wrong on this, but I think most dive operators are not making money hand over fist, at least my friends aren't. They work hard 24/7 for every penny they make and neither they (the operators) nor their employees are making what they should. To suggest they should charge more and pay their employees more is ludicrous in MHO. If given the choice between two dive operations, equal in every way except price, divers will take the cheaper of the two. Having many times volunteered my time on a boat in the Keys (on my vacations for my owner friend), and busted my butt each time I did it, I know first hand that divers in generally are not only not big tippers, most don't tip at all. Many on this board may disagree with me, but what these operations usually see are the vacation or casual diver, not the "hard core" aware individuals who interact on this board.
I have been very blessed and fortunately get paid exceedingly well for what I do for my "real" job, but I would never return a tip for my "volunteer" work! Dah?!?! What I have always done is give it to the captain/owner to disperse among the employees of the dive operation.