The OU was against mixed dancing. Most of the ship’s business came from kosher-keeping Jews who still wanted to be able to dance on their anniversaries and whatnot. The type of economic bracket that would oppose giving kosher supervision to a place that allowed mixed dancing probably couldn’t afford such a cruise.
When and where I grew up, there was an OU member congregation with mixed seating in the sanctuary (this shul also had a pre-selichos dance, strange to say). I found an article from 2005 about an OU shul in Denver that was the last holdout. According to the article, the OU had been trying to “convert” their remaining mixed seating members since the 1980s.
Young Israels used to have mixed dances in the 1950s.
I don’t support mixed dancing, but I think that using it as a pretext for determining the kashrut of an establishment’s food (especially while opposing people who want to have “ethical certifications” to reflect their values) is hypocritical and a poor decision.