I hadn't thought about it, but now that you mention it, getting married onboard makes more sense than in a port.
My bride and I promised each other we'd get married on a ship if at all, and have since decided to get married onboard the Carnival Liberty, upon which we're cruising from Citavecchia, Italy to Miami, Florida Nov. 18 - Dec. 4, 2009.
We wanted to get married onboard as it departed from St. Maarten, but I've discovered that St. Maarten isn't on Carnivals list of places where they offer onboard weddings, so now we may get married in Miami before we disembark.
This change of plans isn't as critical to us, since it's not likely anyone else will attend our wedding, unless we make friends here or onboard.