Same here in SoCal though the more I dive on boats, the more I see what kind of "services" boat crews provide. Some remove fins for divers coming up on the swim platforms while some don't. Some are perfectly okay with doubles/sidemount while others are single tank only. Some are very quick with air fills while some seem to take forever. Some provide pre-cooked food/meals while some make them from scratch.
I also know several boats who charge less (about 40% off) if you sign up as a freediver cause I guess they don't need to give you air fills and help you up the swim platform. A very well-known LDS said that industry standard tips is $5/dive while another said it's the same as the standard 15-20% "restaurant" style guideline. I was just on a charter yesterday organized by a dive shop and they said tipping is 10%. I'm really curious to hear others' opinions on this especially those who dive in California.
This is really the heart of the matter: unlike having a meal at a restaurant, where the process is always the same no matter where you are (you sit, you order, you eat, you pay), dive boat experiences vary widely. When I dive in Key Largo (or most Caribbean locales), the agenda is typically a relatively short (30 to 40 min) boat ride to the first site, dive, SI to the second site, dive, back to the marina. The whole thing usually takes less than four hours.
In contrast, I was on a dive boat last weekend that left Long Beach at 7am for a two hour crossing to Catalina (during which we were served breakfast.) After two dives (at two different sites), a break for a hot lunch, a third dive (at a third site), some dessert, and a two hour ride home, we returned to the harbor at 7pm.
The role of the crew in both of these scenarios is markedly different: in KL, the crew is more "hands on"... assisting with gear, providing weights, hauling tanks on and off the boat, and guiding the dives. In California, everything is self-service... I brought my own tank and weights (hauled all my own gear onto the boat), no guide in the water, etc.
But... a four hour trip vs a 12 hour day (which is even longer for the crew, having to prep the boat before and clean up after.) BUT... I know for a fact that the KL crew works only for tips, whereas the California crew is paid something by the boat op.
My point: a simple "per tank" or "percentage of the trip cost" guideline is tough to apply universally to all situations, because so many factors with dive trips vary from place to place. Consider all of the work the crew performs and recognize that if they weren't willing to put themselves through it, you wouldn't get to dive. Tip accordingly.