For ballroom dancing as far as I know they usually using staff members these days.
The last couple I recall working on a cruise ship doing it fulltime were on the NCL Norway back in 1993.
I have not heard about it much lately - but if I were to look I would try Crystal and Cunard first.
I am the editor, but I also speculate, ask questions and play devil's advocate. I reserve the right to change my mind.