I'm not a working DM, but I thought I'd at least relay one experience I had on a boat that I truelly felt sorry for the DM's.
I was on a day boat out of Port Douglas in Queensland, AUS.
We were going out to do a part of the GBR.
Anyway, most of the boat was booked by a group of Japanese tourists, probably 20 of them.
The boat had 30 or so people on it minus capt. and crew. It was probably in the 40 to 50 foot range I'm guessing, a pretty fast boat.
The Japanese took over the entire back deck and wouldn't let anybody else have room to do anything. The rest of us had to find some nook or cranny to try and get our gear on. No matter how hard the DM's tried they couldn't relay the message that other people needed space too. As the dives went on the deck got messier and messier.
After the third dive there was so much gear and crap strewn accross the back deck (knee high) that I just started walking right on it, screw it!
Underwater they looked like a big dust ball with an occasional arm or fin sticking out of this tornado of Japanese roto-tillers, with one DM in front and one behind kind of guiding this ball of dust, fins, tanks, divers and whatever else made up this storm.
They were all done with their dive in about 15 minutes (first guy out of air), one of them pointing to his regulator that it stopped working! OOA? gee U think?
That screwed the rest of us out of a bunch of bottom time since the rules were to come up with the group. Me and another guy from the states and a few divers from UK were kind of doing our own thing close by and were looking at each other in amazement as all this was going on.
We got very little service actually, the food had been mobbed by them. The boat was essentially taken over and destroyed by them.
On the way back there was a whale that came very close to the boat. The captain stopped the boat so people could get a look. All the Japanese tourists were up on the flybridge where they weren't suppose to be in large numbers. As the whale went from one side of the boat to the other they would all run accross the side with the whale and I saw the DM trying to physically get some people back to balance the boat out because he was concerned about a roll over.
Finally after it was all over and we were back at port, NOT ONE OF THEM put a single dollar or a dime into the tip jar!!! Not one.
Can you imagine!
I asked the poor DM if this usually happens and his reply was "Dude, you have no idea!"