You'll find most posters here will state they always tip over and above what the cruiselines recommend, along with tipping for everything that anyone ever might have done for someone at sometime.
They'll tip people 10 & 15 for room service, tip the dining room staff even if they never eat there.
They'll also recommend that you tip in the alternative dining, even though it has a service charge.
There are even some who will state that you should tip even if you don't get good service and have to keep repeating a request to your steward over and over.
If you don't tip then you will be classified as cheap, mean spirited, etc, just beacuse it is the way they see things. It doesn't matter if their culture isn't one for tipping.