Cunard line (QM2, QE2, QV) have a ballroom which is the centerpiece of each of their ships.
They include daily at-dea ballroom lessons and dancing to a live orchestra each night. A professional dance couple is on board to provide the lessons and are available for privates.
Four nights out of seven are balls (ie Masquerade ball, Nautical ball, Captains ball, Black and White etc) where dress code is formal.
The ballrooms are beautiful. They even have afternoon tea dances each week.