Not really sure how Carnival does it, since my only sailing with them wasn't on my card, but when I was on Costa (which is a Carnival Corp company), I saw several holds on my card. They totaled up to a day or two of my use on my Costa card. The 2 days at sea I used xxx dollars on my account, and I had a hold for that amount placed. Day 3 was USVI and I had access to my banking since I had full Sprint coverage and didn't realize it when I was looking at balances (Otherwise I would have bought more art, since I saw my avail balance and did on ship usage math from there). When I was in PR
the next day, I saw another hold for what I had spent since the last hold.
At no point did I see a large single hold ... just the few little ones.
I know its a different cruise line, but its a Carnival Corp line, so would think at some point they would standardize on things.