Our family has always cruised alot. My daughter just graduated from University of Florida this past June. Her dream for the past couple of years sincing sailing primiarly Carnival has been to work for Carnival when she graduated. Oddly enough through Twitter, she made some key contacts at Carnival which were fruitful in allowing her to "job shadow" accountants on board this past May. One thing led to another and now she has completed her month long "Accounting College" and is on her first contract on the Ecstasy out of Port Canaveral as Shipboard Accountant.
The previous posters are correct, you will not find many if any Americans at all working on board. Many hired Americans have not worked out in the past, so they are very reluctant to hire them. The pay is very low, but when you consider room and board provided and no "at home" expenses, the bottom line is very similar to if she were to be working at Corporate in Miami. That is her goal one day and they hire many people who have worked on board because by that time they know much about the business operations.
I urge you to follow your dream, and take the advice of the others in terms of what to study for CD. It's true you could start in the Purser area with that background and then work your way to cruise director. It may take a while, but it's worth it if that's what you want.