Cruise lines have to move their ships around the globe each season. No one wants to cruise the fjords in January, right?
Moving that ship from Norwegian waters to Jamaica is a long, expensive proposition. It pays to have paying customers aboard.
These are known as repositioning or "repo" cruises. They tend to be advertised very little, because many of the people who book them have a standing reservation every season. Word of mouth fills a lot of these cabins.
But some are advertised, and there are available rooms if you plan ahead several months or more.
Understand that the total cost of these trips is often larger than a traditional one or two week cruise. Some last three weeks or more, so naturally there is a higher price tag involved.
But the daily cost often is very attractive for repo cruises. You'll have more time at sea to enjoy the ship's amenities, and many lines stage special programs on these trips. You'll also see some unusual ports that normally don't receive cruise traffic.

