to take a child out of the united states you must have written (and notarized) permission from either the custodial parent (as determined in a court of law) or in a 2 parent household both parents.
you may get different info. from a cruiselines people but you must remember that the requirement is necessary to meet governmental laws (and the cruislines policys may be far less restrictive).
also if you are traveling with a child other than your own (any place, in state/out of state/out of the country)-take the time to have the custodial parent (s) fill out a paper and have it notorized that authorizes you to have medical power of attorney (Limited and Revocable on the parent's part) for the child. also give the adult copies of the childs medical insurance card, name of their primary doctor/phone number, any allergies, prior medical conditions, surgeries, and a list of any and all meds they have taken in the past few weeks/will be taking on the trip...this can be a godsend in securing treatment for an ill or injured child.