As a charter operator gratuity to my deckhands is both customary and appreciated. Its just like a restaurant 20% should be the norm for good service. I know thats what Itip when I'm on someone else's boat. If you don't feel you recieved good service please speak with managemnt so the situation can be addressed! Without knowning more about the charter it is impossible to evaluate whether that is a good deal, BTW. How big of a boat is it, how many other divers, level of service provided, ect ect.