I just came back and we took DS best friend with us...I printed the form out like the one in prev. post, filled it in, the parent signed and notorized it. I never showed the letter to anyone. Not to say that they won't ask. We have taken our DS and DD (both from prev. marriages, one to each of us) on previous cruises. I have never had to show the letter from our ex's to anyone. I have always had them just in case.
One more tip...get a copy of the child's insurance card...just in case.