This is difficult to answer b/c I've had personal experience both ways.
For years, I've cruised with my children without their father and always carried a signed, notarized letter from their father. However, last year I did not and wouldn't you know it, we had difficulties at the pier upon check in from NCL! I had to call my children's father from the pier and get him to fax some kind of written consent to take them out of the country. I have a friend who could not board a plane to Mexico with his daughter b/c he didn't have written permission from her mother.
So while the odds have very much been in my favor of NOT being asked for paperwork (we cruise many times a year and were only asked once for the paperwork), it was a hassle the time I was asked and did not have them. The main reason they ask is to reduce kidnapping kids into a foreigh country.
It's totally your decision but perhaps you want to consult a lawyer so that you can get a document that you can use each time? You may find what you need at: www.forms4parents.com
Family Cruise Editor