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The first thing you need to do is contact the cruise lines wedding department. They have packages for getting married on the ship the day of departure, or you can arrange a package to get married at one of the ports. You need to make sure the date you are looking at is open. They only allow a certain number of weddings. If you are looking at Valentine's day I would get it booked early. You will have to have your cruise booked and a cabin number before they will book the wedding.
The wedding department will go over all the details with you and explain everything . Packages can include flowers, cakes, music, etc.
You can arrange to have an officiate of a specific religion if you so desire. I have attended a couple weddings on board before departure in Miami. They were done nicely.