Employment Contract terms vary by cruise line.
They also vary by position.
NCL America is the odd one out, as they cannot afford the astronomical insurance rates that would result from all their Ameircan employees claiming unemployment insurance between contracts.
NCL America is forced to pay all it's employees 12 months per year.
Most of the mainstream cruise lines pay their rank and file employees only when they are onboard the ship. Most of these lines include vacation pay as part of their regular salary.
When most of these rank and file crew go on holiday, they are not paid anything at home.
Officers are treated differently on several of the lines. Their contracts are open-ended. They are usually paid 12 months per year, and are contractually obligated to be onboard a ship a certain number of days every year. Their schedules are subject to frequent changes and adjustments. Their salary also includes a premium for vacation pay.