Kim,
I am not an expert on the subject but on the 10 or so cruises that we have been on with dance instructors, they usually had a couple so that both parts of the dance could be shown. The exception was on one cruise where the regular instructors did not show up and they had two young girls from the Cruise Director's staff fill-in. They only knew very basic steps and we were not impressed. Good luck with your pursuits. I would try contacting the cruise lines directly to their home offices and they will either answer you or give you the name of the talent agency that they use to fill these jobs. Please look at operabob's postings above for the names of the cruise lines that seem to be more interested in dancers. I would not recommend that you try RCCL, NCL or Carnival.