Double check with the cruise line, but I think you do need to have a notarized letter from the parent who is not going giving you permission to take your child out of the country. If your cruise stops in Mexico you will definitely need it. It doesn't matter if you are happily married or divorced, you will need permision. No one may ever ask to see the letter, but if they do and you don't have it, you will be denied boarding and you will not get a refund of your fare. Even airlines may require it now.