Here's my perspective on tipping as a crew member on a NJ dive boat. Not to belabor the point about "not doing it for the money" but here in NJ you can be sure your dive boat crew members are "volunteers" who work a full-time job unrelated to diving during the week to pay the bills. Also keep in mind that NJ dive boats are truly a "labor of love" and that the captains who own these boats are lucky to break even on their operations, so "paying the crew a decent wage" is not an option, unless you're willing to have the price of your charter jacked up by much more than the $20 you might tip.
As for me, my Monday-Friday "surface interval" gig pays pretty well. But for some crew members any tips we might receive determine whether they can afford air fills, gas, tolls, etc to crew next weekend or not, but I think for all of us the financial aspect of any tips we receive is no where near as important as the simple recognition that our time and effort has some value and is appreciated by the passengers.
What is that time and effort composed of?
On any given weekend from April-December we happily wake up at 3:30am on Saturday to check the marine forecast to determine if we should get out of bed at 4am to drive 50 miles to get to the boat at 5am, picking up food, soft-drinks and ice on the way, so we can spend an hour getting the boat set up by 6am so that when you get there at 7am we can help you load your two sets of steel doubles, both your deco bottles, your giant tote box of gear, your wet/dry suit and two other bags of stuff on the boat so we can leave by 8am, securing your gear so it doesn't go in the drink, so we can get you to the wreck by 9am, spending the last half hour of the ride in the sweltering heat fully-geared up in a drysuit, fighting back our own seasickness so we're ready to jump into the 50F water as soon as the captain says "GO" so we can rocket down 130 or so, solo, in the dark, fighting current, while dragging 50lbs of shot/chain, and in low viz locate the safest spot to tie in to the wreck and set a strobe for you, as quickly as possible so you don't have to sit in the sweltering heat, fully geared-up, puking all over yourself. After potentially burning much of our back-gas doing that, we get a short dive in and are often getting back up and out of the water while many passengers are still getting geared up. And while youre doing your first dive we're hurrying out of our gear so we can hose down your puke, hang your gear line, straighten the boat up and put lunch and soft-drinks together so that when you come up from your dive we can take your spear-gun/camera/catch bag, unclip your deco bottle, and safely get you and a dozen or more other divers in steel doubles out of the water, remove your fins, get you over to your seats, tanks bungied in, out of your gear, stow your deco bottles, put your catch into the game cooler, serve lunch, and do whatever else we can to make sure everyone is having a good time. During the surface interval well help folks switch over their tanks if they want, provide an o-ring to a passenger who needs one, pull an extra mask strap out of our own sav-a-dive kit for another diver, and maybe lend an extra layer of drysuit undies to the diver that was really cold on the first dive. Over lunch we'll tell everyone where to find the 5lb lobster we didnt have time to nab because we blew through 1,000psi on the tie in or maybe one of us will pull out a binder of deck plans to show you how to get into the best spot to find artifacts on a wreck sunk by a WWI or WWII U-boat. When everyones ready, we help get all the divers geared up again, pointing out un-connected drysuit inflator hoses, dunking masks in the rinse bucket, and getting everyone back in the water for dive number two. While everyone's under, we straighten the boat up and then sit around for an hour, wishing we were doing a second dive. (We often get to pass the time by unclogging the marine head, cleaning the macerators, and putting them back together.) When divers start coming up we help everyone back out of the water, get out of their gear, and stow more lobsters/fish while one of us quickly gears-up to do a solo bounce-dive to 130ft, untie quickly, do a free ascent down current in the open ocean 30mi from shore while waiting for the 50' boat to do a live pick up while slamming up and down in 3-5's. (Depending on the charter and the needs of the passengers the crew member who unties is either the lucky one who gets to do TWO dives that day or the unlucky one for whom this is their ONLY dive of the day.) Out of the water and quickly get out of our gear so we can pull up 160 of line, by hand, including 50lbs of shot/chain, get everything stowed, and get underway, so we can get you back to the dock quickly (while helping folks shuck scallops and/or drag mussels on the way back in) so we can help you get your two sets of steel doubles, both deco bottles, giant tote box of gear and two other bags of stuff off the boat so you can leave to get home early while we spend another hour and a half cleaning up puke, fish guts, sediment, soda cans, lunch leftovers and other general grit and grime, re-fuel, close the boat up, and drive 50 miles home, typically making an extra stop by one dive shop or another to drop off the lost-and-found items passengers inevitably have left behind so they dont have to drive 100 miles back and forth to the boat during the week to retrieve them. If there's no traffic (there always is in the summer) we can get home around 6pm, having spent $20 on round-trip gas/tolls and another $20-$30 on tank fills, and having told the passenger we gave our own replacement mask strap to not to worry about it. After rinsing our gear and deciding whether to try to wash the puke and diesel out of our favorite t-shirt from Cozumel or just throw it away, we get to have dinner and go to bed early so we can get up at 3:30am on Sunday and do it all over again. On a decent trip, each of the three crew members on the boat that day might walk away with $60-$80. After paying for gas, tolls, air fills, etc that's about $20, which works out to less than $2 an hour.
So the intellectually lazy among you can now post "Why should I subsidize your career choice" or "It's not my fault that the captain doesn't pay you enough" or "I already paid for the charter, why should I tip you?" but since you KNOW the crew works for tips keep the following in mind:
A heartfelt thanks with a smile and handshake with or without a few bucks says Dude, I really appreciate what you do!"
Slinking off with no acknowledgment whatsoever financial or otherwise pretty much says F--- you, you should do all that for me for free!
:coolingoff:
Of course, I could be totally wrong, they just may be completely strapped for cash or even ignorant of tipping. In which case I would be wrong. But I would be dishonest to not say what my reaction would be. I'm human! 
