Well, I might as well chime in here....
First of all, I NEVER ask for tips on the boat. I run a 6-pack charter boat and often have one group on board and it seems like begging to mention it. If someone asks, I tell them that while it's not mandatory, it is appreciated if we've done a good job and they enjoyed the trip. The "standard" seems to be about $5-10 per diver per half day.
One thing I do want to say from a DM's point of view is that we do more than briefing and hauling tanks. We're also prepared to do field repair on your gear, get you a band-aid for a cut, and generally make sure that if there is an issue of some sort on board that it won't impact your ability to dive. These are things that most people don't see because nothing happens, but that preparation is an important part of the job. We're also constantly assessing the dive conditions, the divers, and making judgement calls all day as to what sites are appropriate for the divers on board and whose bubbles to watch for problems. We're also prepared to jump in the water and put ourselves at risk should anyone on board need help. We also have insurance to pay and certs we need to keep updated. There's no complaint here, but if your trip was smooth as silk and seemed to run like clockwork and you had a great time, thank a DM.
As for instructors, often they aren't making more than $150 an OW student and that's a day of pool and classroom and four open water dives. If you count out the hours, that's not a lot of money if there's only 1 or 2 students. I know some people don't tip instructors, but if you got more out of the class, especially continuing ed like AOW or a specialty than a card or a few guided dives, the instructor has earned a tip, IMO.
Let me be clear on this, not every DM, captain or instructor has earned anything above and beyond what the shop is paying them, but many do go the extra mile to make your dive experience the best that it can be and for that extra effort, they deserve some extra money.
Rachel