Do not tip at the time of your engagement. If you feel strongly about the level of service and you want to tip - and you will - prepare a separate envelope for the head waiter that served you that evening and present it to him at an appropriate time on the last day of your cruise. If you like, you can write a note suggesting that he distribute portions of what you provided to others that you specifically name (e.g., the assistant head waiter) or you can simply provide additional envelopes addressed to the persons you wish to tip. Make sure you ask the waiters name when you are leaving and then write it down so you don't forget it. Same for others you wish to tip if you do it that way.