You can have a wedding at most of those ports. Check out this company
www.aweddingforyou.com
They arrange weddings in San Juan, Nassau, and St. Thomas as well as many other places. They give the bride and groom transportation, provide and officiater, and have champagne and a cake afterwards. Make sure you check the marrige licensing details.
If you decide to do that, check with Costa and they may be able to provide a private party for you. Of course it will cost extra, but worth it.
A wedding for you can help you along the way. Best wishes to you!
Pat