I never tip ever even in land based attractions. Never once has someone gone beyond what was expected and therefore by definition earned a gratuity. All this talk about how little the ships employees are being paid just infuriates me. Well as the US dollar is worth much more in most of their home countries, and they are getting room and board. The cruiseships certainly are not hurting for employees and no one is forcing them to take their job.
Don't feel bad because you don't tip. The expected practice in this country is WAY WAY out of hand.