There are times and circumstances where there may be an advantage. I work for and American agency but we book for people all over the world. I have many Canadian clients, the charge made at the time of booking is converted at that days exchange rate. When the payoff is done the same process occurs. With the U.S. dollar's recent drops, the CA dollar is actually at a positive exchange rate for the first time in decades. Paying off a cruise right now represents some significant savings over what it was a short time ago. If the trend continues, the value will continue to increase. If suddenly the US dollar rebounds the saving would be gone. Same situation with the pound and euro right now. Very attractive exchange rates.
If you are in a counrty outside the US and booked with a US T/A, you have some speculation to do. Is the dollar going to continue to drop? If you think so, don't pay off your cruise until the final date. Will the US dollar rebound? If you think so, pay off now. There could be some serious saving if you guess right.