Here are some recommedations
http://www.cruisemates.com/articles/feature/cruiserefused-062708.cfm#axzz167FJKggE
Minor Children Traveling with a Legal Guardian
Adults who are not the parent or legal guardian of any minor child traveling with them are required to present the child's valid passport and visa (if required) and an original notarized letter signed by at least one of the child's parents. The notarized letter from the child's parent must authorize the traveling adult to take the child on the specific cruise and must authorize the traveling adult to supervise the child and permit any medical treatment that might be administered to the child. If a non-parent adult is a Legal Guardian, the adult must present a certified certificate of Guardianship with respect to the child.
Take note that if the child has two legal parents and one is not going on the cruise, even in cases of divorce where only one parent has legal custody, it is a very good idea to get notarized permission from the parent who is not going on the cruise. Immigration officials have been known to deny a parent taking a child out of the country without the absent parent's notarized permission slip.
and from Cruise Diva
http://cruisediva.com/parental_permission1.htm
For single (divorced, widowed, or simply married, but solo) parents, grandparents, or family friends taking children on a cruise, there is an often overlooked planning step that can end a vacation before it beginsthe permission letter.
Airlines, cruise lines, and immigration agents can deny minor children initial boarding or entry to foreign countries without proper proof of identification and citizenship and a permission letter from absent or non-custodial parents.
According to Department of State Publication 10542: "With the number of international child custody cases on the rise, several countries have instituted passport requirements to help prevent child abductions. For example, Mexico has a law that requires a child traveling alone, or with only one parent, or in someone else's custody, to carry written, notarized consent from the absent parent or parents. No authorization is needed if the child travels alone and is in possession of a U.S. passport. A child traveling alone with a birth certificate requires written, notarized authorization from both parents."
Proof of identification and citizenship is relatively simple to obtaineither a certified copy of a birth certificate or passport. The permission letter is a bit more vexing since most people aren't aware of the necessity to have it, let alone what it should include.
An attorney could prepare a formal affidavit, but a simple letter-style document is adequate as long as it is signed before a notary duly authorized to administer oaths. To be acceptable, it should include specific details about the trip, the custodial adult(s), and the child(ren). While no one wants to think about medical emergencies while on vacation, it is also wise to include consent for the adult to authorize emergency treatment for the child in case the need should arise.
Some parents, particularly mothers who don't share the same last name as their children, take no chances and also carry a copy of their divorce decree or, in the case of widows, a death certificate.
After going to all the 'trouble' to secure proper documentation, it could turn out that no one even asks for it. Why did you bother? Because if you hadn't, the possibility exists that your cruise ship may have sailed without you and your very disappointed family. You may even find that it's easier to get into a port of call than to leave it with your own child!
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