I don't need a divemaster when I dive off of my boat or shore dive.
So what your saying is you don't tip when you go out to eat cause you could have gone to the kitchen made your own food, brought it out to the table, then cleaned up your own mess afterwards.
Based on the exact same logic you don't need a boat captain to drive the boat cause you can drive your own.
Sure you may not "need" them but if you didn't want the connivence of having them there you wouldn't be on someone else's boat to begin with.
I still think they should be payed by their employer, the employers are the ones that require them.
In regards to the employers being the one's who require them this is actually not true PADI authorized dive boats that carry more then six paying passengers must have a Dive Master on board. For those holding less then six paying passengers the boat captain must be a Dive Master. So no the employer is not the ones that are requiring them its the insurance companies that require them. This is not a coast guard requirement. If you think about the statement of employers require them then you would realize that in this economy there is no way a dive shop would put an employee on the boat just for the sack providing you with an extra level of customer service that you have stated you could do yourself anyways.