Never did a future cruise credit or booked a cruise onboard while cruising with Carnival, but have on other lines. There is one caution that should be noted.
If you are on a long cruise be sure to book near the end of the cruise. I once bought a future cruise certificate on the 3rd day of a 28-day Princess cruise. They charged my credit card that day and when I got home the credit card bill was there with the payment date already past
. I assumed it would be charged to my sea pass, but was not
. Since Princess & Carnival are the same company, I assume Carnival would do the same as Princess.
O F C'er