I researched the same thing when we were planning to sail out of Miami. I googled a few wedding planners (search for 'cruise stop weddings') and got some really good quotes. It all depends on what you want to add to the package. They will do all the legal stuff for you and take you back and forth the ship.
We then changed our plans and are sailing out of Barbados. I've got a wedding planner helping me with a simple wedding on the beach before we set sail. We've got a few extra things included and it's costing us £700.