My wife and I sailed on the Crown Princess two years ago with one of the ballroom cruise providers. Although there was no provision made for private ballroom dancing per se, a subgroup of us "took over" one of the lounges in the evenings. There was a quintet playing there after dinner who played nothing but ballroom once they saw there was a group of serious dancers who appreciated and actually danced to ballroom music. The floor was rather small but adequate if you kept your traveling in check.
Unfortunately, so many passengers came down with norovirus that the cruise was cut short and we returned to Lauderdale 3 days early after sailing almost to South America with no stops. (See "Princess Noro-Cruise to Nowhere" on cruise boards.)
Princess did give us a full refund and paid our airfare home, but that was our last cruise. It's not worth the risk to us of having a pricey vacation ruined being trapped at sea while our fellow passengers are vomiting in the hallways. And in the elevators. It can happen on land too, but there you can at least crawl to the beach to die.
The saving grace of the cruise was the dancing. Although one instructor got sick and we didn't see her at all after the second day, the remaining instructor was outstanding and he made up for a lot of the hassle.
Your experience may vary, of course...