As Donna said, when doing something like this, it's very important to find a good reputable agent you can work with who has the experience and resources. (For example, we have special wedding packages available directly through our company.)
Having done this several times, I can tell you that there are alot of questions and details that need to be worked out and it's important to find the right cruise based on the requirements.
Some of the considerations you need to think about; how long do you want to cruise? (Keep in mind that with some exceptions, most of the bigger and newer ships are going to be doing 7-night cruises.) Where do you want to cruise to and from? (This will have a big impact on options.) When do you want to cruise? (Some cruise lines move ships out of the Caribbean during the Summer and relocate them to Alaska or Europe.) Do they want to get married on the ship while underway? (Very few cruise lines allow this.) Do they want to get married at the embarkation port so some people can attend the wedding, but not go on the cruise? (This is great for those who may not be able to afford the cruise, but still want to attend the wedding.) If they want to get married at a port of call, which ones are they considering? (Keep in mind that ports have different requirements concerning marriages - i.e. length of stay, paperwork, etc.)
Planning a cruise wedding is like buying a car; there are alot of options available and you need to decide if you want the basic model or one with all the bells and whistles. And then you have to worry about all the guests and make sure all their demands are met.
Needless to say, you can begin to understand why it's so important to get the proper help.
__________________ Travel Agent/Cruise Specialist w/12 yrs exp and 47 Cruises on 11 cruise lines! Favorites: Paul Gauguin - Tahiti: Uniworld River Cruises - Europe; Celebrity Solstice-class ships; Holland America - 12-nights Baltics & Russia; RCCL - 14-nights Greek Isles, Turkey, & Croatia; Holland America - 14-day Alaskan cruisetour; 10-night Canada/New England cruise; 21 days Hawaii w/7-night NCL cruise; Oceania - 25 days in Asia; more than 3 months touring Europe by train. And many all-inclusive resorts!