For a day boat charter, from The Scuba Snobs Guide to Diving Etiquette, Chapter 3.
PLEASE TIP
Do it. Tip something. Tip in local currency if you can. Always tip in cash. If you are clueless as to what is appropriate, then tip five to ten dollars per tank per diver. Thats a fair range. The tip can be on the lower end if the services provided are minimal, more if someone set up your gear for you, changed out your tank, gave you a beverage and/or snack and/or lunch. You should tip more if the divemaster retrieved the piece of gear you dropped overboard, or saved your life, or did something else special like that. Some boats have a tip jar. We make a habit of asking if there is one, and usually ask loud enough so other people will hear us ask and hopefully tip. If there is a tip jar, put your entire tip in it. If there is none, tip the divemaster and captain separately. Also, tip every day since the crew and dive masters may change daily. But TIP. However, tipping is excused if the boat abandoned you at any dive site.
DivemasterDennis