I can provide what I think is an interesting perspective, as someone who is a frequent charter passenger and now newly minted crew member and DM-to-be.
Before getting a spot on a crew I always understood that that "they don't do it for the money" but now I have a better understanding of ALL THE STUFF THEY DO that they don't do for the money. (HINT: Our crew just spent 3-4 full days with no pay getting the boat ready for the season, repairs, sanding, glassing, painting, cleaning, waxing, etc)
Crew also...
- gets to the boat at 6am so it's ready when you get there at 8am
- is there cleaing the boat for quite a while after you leave
- helps you gear up and get in and out of water
- regularly practices rescue scenarios, man-overboard drills, fire drills, etc
- is willing to jump in the water and save your @ss if needed
- will lend you their own gear or any other help you might need
If you're diving on a six-pack and there's two crew members for 6 divers who each tip $10 that totals to $60. Typically the crew is not paid other than tips so split that two ways for $30 each. Not bad, huh? Well, think about it this way: that doesn't even cover the crew's tank fills and gas to drive to the boat. So not only are they "not getting paid" for busting their butts to help you, it's actually costing them money to do so.
Not playing the sob story, because we willingly do it "for free" but the one thing I can tell you is that while I used to tip pretty well BEFORE...now that I know what all goes into being crew I tip even BETTER!