What is the customary tip for dive charters?

[g1138]

It's called Emotional Labor. You don't pay a dive boat for a social friendly chat with the crew, you pay for safe transit to and from the dive site.
So if I enjoyed my transit because of the friendly crew, I'll tip them.

 

[dierenarts]

My son did the PADI E-Learning thing and we're now on an island. What would be customary for the instructor doing the confined and open water instructional part of the course?