It doesn't seem to be universally understood that a DM basically works for tips. When I first got in the boat business, I worked as a DM. Get to the boat at 6:30, carry 20 gal. of ice from the rest. to the boat, get bag of oranges to cut up for SI, load 90 tanks making sure that required number were nitrox and properly labeled, wipe down seating areas, fill water tanks, help passengers on board. On the way to the site give boat briefing, at the site give site briefing. Help set up and breakdown gear for those needing such help. Help in and out of the water also for those needing it. When needed act as counselor, mentor, educator and entertainer. Be prepared to sacrifice myself to help a perfect strange at a moments notice. In general, pay in the range of about 3500-4500 in gear and training to get the job. Clean the boat from any sea sick passengers and the domino affect they cause. Make sure the marine head is working properly and doesn't look and smell like a science experiment. (Even though I told you nothing goes in the head that didn't pass through you, someone still manages to plug up the head 3 times or more a week.) When we get back, unload 90 tanks from the morning trip and load on 90 more for the afternoon trip. Give the boat a good going over before the afternoon passengers. Pass off to the afternoon crew and start filling tanks. This was all for 35 dollars and tips. The whole thing took about 6 hours plus fills. I got paid 8.50 an hour for filling. That was in the days of 45 dollar 2 tank dives. Now the same dive is 100 to 110 and the DM still makes 35 or so a trip.
A live aboard DM generally makes about 100 a week plus tips. Most people thought we made about 350 a week. I hear one company only pays 75 a week for DM. I've actually heard one resort owner tell a DM part of his pay was all the sunshine. Too bad the bank doesn't take sunshine for his car payment or rent.
A good crew works hard for the guests, is well trained, makes the whole thing look easier than it is and works as a team. They have no control over the weather, current, waves, number of sharks, mantas, whales, etc... you may or may not see on the dives. Generally a crew splits tips equally. The exception, at the time, was if the Captain was the boat owner, he got none of the tip pool.
I'm always shocked by the number of crews that practically beg for tips. If you do your job WELL, the people feel like you added positively to the experience and usually show some appreciation. Just a side note. To any of you that leave all your extra booze as a tip for the live aboard crew, they are forbidden to drink for the duration of the rotation, usually 6-9 weeks. What is greatly appreciated is to leave any extra batteries, mag. or books and DVD's.
As far as the custom of tipping goes. As divers, we get to travel and interact with cultures from all over the world. I try to make it a practice to learn as much as I can about the area I'm going to. I always go by the When in Rome mentality. I've been known to give the shoes off my feet and the shirt off my back. (It's amazing how much a pair of Nikes or a sports team tee shirt is worth in other places. )