I wrote up a letter similar to that, listed our names, address, SSNs, trip itinerary, contact info, etc. The child's mother is signing it and having it notarized. We carry my stepson on our health insurance, so that is a plus if we do need to use it.
I understand about the problem with abductions and quite frankly I am surprised that the cruise lines do not have a more concrete policy regarding situations like mine. You'd think they would want to make sure their own butts were covered before allowing a minor to travel.