You will have an assistant waiter as well as your main waiter. And you will have a cabin steward. These people are the main individuals assigned to your service.
Some of these people may not be Christian nor celebrate Christmas. If they do, then money is always the best option. A lot of people give them calling cards thinking this is a novel idea they would appreciate, but the waiters I've talked to said money is preferable to calling cards, and they have calling cards they haven't used.