It depends where in the world you are diving. Do you mean the US? There are some places in the world and styles of diving (e.g., many liveaboards) where dive staff expect every guest to tip and to tip a certain percentage of the cost, other places where it's common for the amount of the tip to vary considerably from one diver to another, and still other places where dive staff are tickled when they receive an occasional tip. In some places in the world it can also sometimes depend on what kind of clientele frequent the dive op.