I live in the Philippines and learned to dive here, so I've only observed Filipino habits.

When we go to resorts, we leave a tip for the resort staff as a whole, or for the dive staff and the resort staff separately if that is they way they have it set up. For day diving, I tip the boat crew. I prefer to tip at the end of our stay because I don't want to have to find the right sized bills for every day of diving and I don't want to feel obligated to tip every day, even if it is a problem day. At the end of the period, I can leave a good tip and feel comfortable about it.

Filipinos tend not to tip for service; generally, boat crews are happy for any tips and we get a lot of good will, even though our tips aren't large.d

Regards,
Bob