From everything I have read, it needs to include the dates of travel, the ports of call, and the non-travelling parent's permission for said child, (name here), to travel with custodial or non-custodial or joint custodial parent, (name here). It should also be notarized. Yo may want to check cruisecritic.com. I have seen some links to sites that have the forms a person can use - no lawyers required.
Given all that, I have also heard it depends on which cruise line you are taking. I have sole custody of my kids and was told by some board posters that I would need a letter as well. I asked Royal Caribbean and they said no I wouldn't. I didn't want any problems, so I got their passports, brought along my custody papers, my marriage certificate (to my new husband), driver's license, and divorce decree. I was as prepared as I could be short of a letter from my ex because he would not have done it nor did I want to alert him I would not be at my house. (Messy situation). I was never asked for anything. Maybe I just got lucky or maybe the ports we visited didn't require that. My children's last names are different than mine.