After boarding, I asked (out of curiosity) how could I adjust the amount of gratuity if service was not as expected. The response was somewhat surprising....I was told that I couldn't do this on the ship...that I would have to contact Carnival in Miami AFTER the cruise to try to get a refund. Fortunately, no action was needed on my part as service was better than expected, but I found it strange
that Carnival encourages you to pre-pay, but doesn't offer any ship-board recourse if things go wrong during the cruise itself.
The reason for this is because the ship has their own accounting separate from Headquarters. The ship can't refund something that wasn't in their accounts to begin with.