Was reading up on the USS Iowa and came across this quote:
Iowa completed her presidential escort mission on 16 December by returning the President to the United States.[7] Roosevelt addressed the crew of Iowa prior to leaving by stating, "... from all I have seen and all I have heard, the Iowa is a 'happy ship,' and having served with the Navy for many years, I know—and you know—what that means."
What is he inferring?
A happy ship, generally, is one where the crew gets on well with each other, and with the officers. It's one where there are few discipline problems. It's a ship where the junior members of the crew are not bullied by their seniors. In general, it's a ship where the crew as a whole work together efficiently and well.