You can easily claim a price drop up until the day your final deposit is due, because you have not paid the full price yet.
Did you book with the Carnival Early Savers guarantee? If so you should be able to claim a price drop up until the day before your cruise sails.
If you see a price drop you go this web site to claim it:
Early Saver Program | Carnival Cruise Lines
Here's how the Early Saver program works:
When you book a 3-5 day cruise three months in advance, or 6 day or longer cruise five months in advance, you are assured the lowest rate.
In the unlikely event that you do find a lower Carnival-advertised rate* after booking your cruise, we will honor it and adjust your cruise rate accordingly. Simply fill out the Price Protection Claim form and we'll take care of the rest. If you booked your cruise with a travel agent and need to make a Price Protection Claim, simply contact your travel agent and they will submit it for you.
What is the price protection assurance feature?
If, after booking, you find a lower Carnival advertised fare, Carnival will honor it, issuing the difference in the form of a non-refundable onboard credit. The lower fare must be: i) for the same ship, sailing, stateroom category and number of guests; ii) available for booking at the time you submit your request for the lower rate; and iii) a rate you are eligible to receive, if special restrictions apply. Price protection will be subject to the prevailing fees and/or fuel supplement if applicable.
What is a Carnival advertised fare?
A Carnival.com or Carnival advertised fare available to the general public. A Carnival advertised fare excludes group rates, membership programs, charters or other Travel Agent promotions not offered by Carnival to the general public
What should I do once I find a lower rate?
Complete a price protection claim form. If the request meets the price protection requirements, we will process your onboard credit for the fare difference and send you an email confirmation within 2 business days of your request.
What information must be submitted for a price protection request?
Available cruise fare being requested
Where the lower advertised fare was found (i.e.: Carnival.com)
When can I submit a price protection request?
Carnival will accept price protection requests up to 2 business days prior to sailing.
How many times can I submit a price protection request?
Provided that you are eligible for the lower fare, there are no restrictions to the number of price protection requests or the requested amount.