A tip, or gratuity, is a SMALL amount of money given VOLUNTARILY as a token of appreciation for a service rendered. According to Webster's Dictionary, the word "tip" is also considered by many to be an acronym: T.I.P. - "To Insure Promptness" or "To Insure Prompt" service. We tip our servers as a way of thanking them for good service. We might also leave a very low tip, or no tip at all, as a signal that the service was terrible.
That was probably correct once. But now a tip is the living wage ofr the servers on the ships. Yes, the ship often give them health care, uniforms, free room and board plus a small salary. But if the servers plans to make any money to support their families then they must relie on the Tip.
The cruise ships have keep cost low by raising tips. Once a tip was given for great service now you must go out of your way to remove the tip from your on board account on many ships. Why? Because if you didn't tip they would have to pay more to keep good staff. Myself I would rather lower cruise fares and then tip.
I prefer to tip according to service level, but being a restaurant owner I can sympathize with the staff when cheapskates refuse to tip. Whether they make the wages in tips or salary, in either case the people have to make a standard of living acceptable to them in order for them to stay in the position. It is customary in this country for them to make it in tips, so be it. But the customer is ALWAYS going to pay their wages in one form or another.