I may be a bit biased being a crew member on a boat myself, but I tip $20 for a two-tank trip as long as folks are friendly and help me have a fun, safe day. In fact, I tend to advise the crew/DMs upfront that I'm a pretty low-maintenance, self-sufficient passenger and they should feel free to focus their attention on other passengers who may actually be in need of assistance. I'm not shy...if I need a hand, I'll let someone know! Would rather have them ensuring no-one else gets into some kind of trouble that will end their - and my - dive day early. (Note to any op/DM/crew I might dive with - I'll also blend my own tanks, carry them to the boat, help the other passengers gear up if they need a hand, and tote a few empties off the boat when we get back if you don't watch out. Can't help it I guess!)