I hope that this message isn't too late to be of assistance to you. I don't know where you are cruising to but I have a recommendation in order for you to cover yourself in this situation.
While it may seem a pain I can tell you why this documentation is necessary. Because of child abductions by non-custodial parents foreign countries are increasingly becoming more stringent with their admittance policies. This isn't a cruise line policy its foreign policy and the bottom line is that a foreign country can refuse to allow you admittance without the proper documentation. For more specifics on foreign policies see the US Department of State website. The bottom line is you don't want to be refused admittance so have your step-child's passport or birth cerficate in order.
Though it may not be entirely necessary if you want a legal type document that should pass muster wherever you go, my recommendation would be to draft a document which you can entitle something like "Affidavit Regarding Parental Authorization". Here is some very basic language for you. I can't pass out legal advice in this forum but you could find an affidavit document like this from a book at your local office supply store so I see no harm in providing a basic affidavit type form. You MUST have this notarized.
Affidavit Regarding Parental Authorization
I, ___________________, being first duly sworn states as follows:
1.That I ________________________ am a person of suitable age and discretion.
2. That I am the custodial parent (or non-custodial if that's the case) of ___________________, born (date, place) as evidenced by certificate of birth, attached. (attach it).
3.Citizenship/address of custodial parent and child.(and all other contact info)
5.That I hereby authorize (your name and husbands name, relationships to child, address, SS#) to travel with child to (include entire trip schedule land and cruise, dates (indent using outline form if needed)) including but not limited to any unscheduled destinations resulting from cruise lines alteration of itinerary at its option for stated period of time or for a reasonable time thereafter in the event of unforseen airline flight and/or cruise line cancellation/delays.
6.That I hereby authorize (your name and husbands name) to seek and provide any all necessary medical attention to child should the same become necessary in their discretion.
7.That I have personal knowledge of these facts and if called as a witness I am competent to testify to the same.
State of ____________)
County of ___________)
Subscirbed and sworn to before me:
______________________, notary public,
on this _______ day of _________, 2002.
Witness my hand and official seal.
My commission expires:
Known to me:_______ I.D.____________
I hope this helps you. My intention was not to make this more difficult for you as a letter probably would suffice for carribbean destinations but why not have it look legal since this one page (or so) document won't take long to type. If you still have concerns consult your attorney. Most of all just enjoy your cruise with your family.