The rule for taking minor children out of the country is that if only one parent is traveling, they must have a signed and notorized letter at the pier. The letter needs to state that the parent taking the child has agreement of the the other parent to leave the country with the child. The only exceptions I'm aware of are in the event of a death of one parent (you've covered this one) or if there is no second parent listed on the birth certificate of the child.
They don't usually even ask for this letter at the pier even though they are supposed to every time. If they do and you don't have the letter you can be denied boarding. Unless there is another high profile custody case in the news of a parent fleeing the custody hearing by taking the child out of the country, it's not likely to come up or that you'd be denied boarding if you have custody papers with you. I'd get the letter if possible, you never know when they'll crack down on such enforcement again or how strict the customs agent you get will decide to be. It's just not worth the chance of losing your cruise and your money.