I have been both tipper and tippee. Some people are good tippers, others don't tip at all. There is no hard and fast rule. Factoring in service, good or bad, is appropriate. If I was your DM on a day boat in Maui, here is what I probably did for you.
1) I topped off your tanks to be sure you got a full tank;
2) I loaded tanks and other stuff onto the boat;
3) I probably loaded you gear onto the boat;
4) I got weights for you;
5) if you asked me to, I set up your gear for you. If you used our rental gear I did set it up for you;
6) I made sure your air was on and checked you over before you entered the water;
7) I dove with you and made you more comfortable knowing that if you could see me, you weren't lost;
8) if you asked, I took your picture or made a short video of you looking cool under water;
9) I made sure you got back on board safely;
10) I may have retrieved a piece of gear you dropped, or loaned you gear if you had a problem with yours, or put a bandaid on a scrape;
11) I told you what a great diver you are and how I wish every group I led was as easy as you guys.
12) after the dive, I cleaned up you mess, found the item you left on the boat and secured it so when you went to the shop, it was there, and then I off loaded the tanks, rinsed your rental gear, and then went home. The shop paid me just about minimum wage for my time. You tell me, what kind of tip is that worth?
DivemasterDennis