It depends on when your sailing date is. If you were required to pay the full amount at time of booking then I would say you are past final payment date.
The refund policy should have been explained in your booking process. However, once booked, and paid in full you cannot cancel.
There are some alternatives:
Many cruise lines will give a partial refund if you are within a certain time period before sailing. Carnival's policy is:
Up to 76 days before sailing: No penalty and full refund (except for Early Saver Fares*)
75 to 56 days: Loss of Deposit
55 to 30 days: Loss of Deposit or 50% of Total Fare, whichever is greater
29 to 15 days: Loss of Deposit or 75% of Total Fare, whichever is greater
14 days or less 100% of total Fare
Call the person or agency you booked with and see what can be done.