If the captain is dropping a diver to get the bouy, let him concentrate and stay out of his line of vision. Keeping your eye on any diver in the water so he can manuever is helpful. Point to the diver so he can look away if needed.
Realize channels sometimes have different rules regarding sitting on the bow, flushing heads, etc so keep alert for these differences. Try and act normal if in sight of the Coasties.
Zip divers who are struggling, don't be shy to steady someone trying to get a leg into a wetsuit or make their way to the exit point. Don't step on fins, offer suggestions to new divers not condescending cracks that will embarass them. Remember, the new diver who has not figured the head out might be your kid's brain surgeon or your pilot home next week.
Chances are, if they can afford to vacation and dive they might just be an expert at something somewhere.
Offer snot checks casually.
Offer water to puking fellow thrill seekers.
Keep an eye out for masks or items that might go overboard and if you move an item, tell the owner where you put it.
Alert divers if they are getting burned by the sun but don't nag. Offer extra weight to people who splash and look like they are realizing they don't have enough or ask "would you like another 2#'s?" Sometimes I even leave a 2 at the mooring line on the bottom.
Pass on your most important tip to others if you have done the dive many times, even when you are not in charge. "watch for outriggers over your heads, folks!" ...that sort of thing.
Help divers to their tank slot if the guide is not available, even if it is just pointing and moving over.
Underwater, be ready to help a new diver who has lost their integrated weights unexpectantly, to stay on the line for the safety stop. Alert fellow divers if the tank is about to skip town, etc.
Make conversation if you see someone who looks really nervous to see what the deal is and help them feel better. Compliment new divers on something (anything) that they are figuring out and doing right.
Be conscious of cameras and don't drop things on expensive ports, etc. Yes, we are special. We are special because we sold our first born to buy that stuff. Have some respect. The bigger the camera, the more respect, that's just the way it is.
When the captain is docking the boat, stay out of his line of vision and settle down a bit. I usually offer to take some trash when I go or offer to help carry tanks. Always ask because some operators will appreciate it and others don't want the liabilty. Make yourself an asset and you might be given a special price or go for free!
Introducing people, even if tourists is really important. Some people are not comfortable introducing themselves and having another diver to chat with makes their trip more fun.
Oh...don't have a computer that alarms at every parameter, which you then ignore. That really bugs people because they keep checking to see what the problem is.