I figure the range of 10 - 20% of what the diving costs and spread that around between the DM, captain and the boat hands. I tip after each trip as it might be a different crew on the next dive. I also give the tip to the person involved instead just one person to spread around. It only happened once and might have been a language thing, but after an excellent day at Punta Sur, I felt the need to tip a bit more and gave the deck hand a $50 and said for you and the crew. (there were two of us on the dive so really $25 each) I later found out it went only into his pocket. Other well received things I have done is run into the local town and picked up a couple cases of beer and dropped them off at the dive shack for all of the guys that handle your gear, book your dives, etc. Usually very well received.