And there is part of the problem. I don't feel it is my duty to pay anyone anything for doing a good job for me. Silly me, but I have an expectation that if I use any type of service, be it diving or anything else, that I am going to receive good service or they are going to do a good job. Good service should be the norm. Why should anyone have to pay extra for good service. In the past, tips were to be given for excellent or exceptional service above and beyond what should be the norm. Tips were suppose to be earned, not expected. That seems to have changed.
What's really sad is that you feel it is peoples "duty" to tip others for simply doing a good job.
Think of your DM like a contractor that you've hired to provide a service for the day (Even if the only thing that person does is provide a warm body that fulfills the Coast Guards' requirement for number of crew on board). In most parts of the world (US included) they are not getting paid (remember that part. Zero, or close to zero pay) and are doing this "for the love of the game". You can elect to not tip them when they simply "do their job" and at some point they'll be forced to go do something else where someone will pay them. When that happens enough times you'll be left with whomever remains and the service will absolutely suffer.
Stop being cheap.