Try the marco polo they have dance classes also hosts if you are travelling on your own, but be quick before they sell it.
Most ships have dance instrutors onboard but you will have a problem learning more than the basic steps for ballroom, latin and salsa. I would suggest that you go to a local dance school where you live and learn the basics and possibly intermediate dances and then really enjoy the dancing on board. You will probably find that the beat dancing opportunities are on P & O and I know that the instructors on there are first class. Ida Dow.
QE2 does dance lessons etc and also have hosts, but again be quick before she goes.