Cruise lines reposition their ships from Alaska to the warmer waters of the Caribbean in the fall, and then move them back to Alaska in the late spring. Most cruise ships that spend their summers in Europe will cross the Atlantic in the late fall to spend the winter months in the Caribbean and then reverse the process the next spring. Other ships traveling to Asia, South America, Africa, or Australia will also often reposition to other parts of the world when the weather changes.
Rather than sail the ships without passengers, cruise lines discount these "repositioning" cruises. Cruise ships must discount the fares to make them attractive to passengers who prefer port-intensive cruises. Cruise lines also recognize that most passengers will spend much more money per day in the casino, bars, and shops on a ship at sea than they do when the ship is in port, so are willing to sail with discounted fares to fill up the cabins.