Are you an American citizen? If you are, I'd recommend applying with NCLA. If you are a foreign citizen, any of the others would be good to apply at.
There are many companies our there making it seem as though the only way to get a industry job is to work with them. While it *may* help, you only need to submit your resume to, or contact the human resources department of each line. Keep in mind though that certain onboard facilities, such as the spas and shops, are subcontracted by the cruise lines to third parties. For jobs at these places you're best bet would be to contact these companies directly.