PandaBear,
For a waitress/chef combo in a restaurant on Cayman--or anywhere else, I guess--10-20% is the range that covers most comfort levels--from parsimonious to generous. A two-tank dive in the Caymans is about $70...i.e., $7-$14 per person per two-tank dive is the the range that puts DMs/boat captains on a par with waitresses.
I hope that any waitresses or DM's out there do not take any offense--just trying to set up the algebraic equation!
Barbara and I tend to discuss how helpful the DM was, if he went out of his way to "find" unusual fish, and all the other things that make a dive better than average...then we tip accordlingly--high for above and beyond, low to nothing for just filling a space.
We tend to be generous because we really appreciate enthusiastic, knowledgeable dive masters!
Joewr