Passengers are often surprised by how much of their credit line is tied up during a cruise. The mechanism is authorization holds, and it is deliberate rather than a billing error.
The ship extends credit for the whole voyage
Onboard purchases are charged to a folio and settled at the end, so the ship is effectively lending across the voyage against the card on file.
To protect that exposure the ship obtains authorizations from the card issuer as the balance grows, in increments rather than one large amount.
Each authorization reduces available credit immediately even though no money has moved and no charge has posted.
Holds do not disappear at settlement
When the final bill posts, the earlier authorizations should drop off, but the timing is controlled by the issuer rather than the cruise line.
Some issuers release holds within a day and others take considerably longer, which is why available credit can stay depressed after disembarkation.
The passenger sees this as a duplicate charge, but the posted transaction and the outstanding hold are separate records for the same spending.
Debit cards behave differently
A hold placed against a debit card removes funds from the account balance directly rather than from a credit line.
That money is unavailable for other spending, including at ports of call, until the hold clears.
This is why lines generally recommend a credit card for the onboard account, and it is a practical rather than a commercial recommendation.
Foreign currency adds a second layer
Many ships bill the folio in dollars regardless of where they are sailing, which removes conversion from onboard spending for an American passenger.
Ashore, the same traveler faces the usual currency questions, and a card with no foreign transaction fee handles port spending better than the onboard card does.
Gratuities and service charges are usually applied daily to the folio, which is part of why the authorized total grows faster than passengers expect.
Managing it before you sail
Registering a card with more headroom than the anticipated spend avoids declined authorizations mid-voyage, which are awkward to resolve from a ship.
Reviewing the folio partway through the cruise catches errors while the charge is still fresh and the venue can confirm what happened.
Cash can be deposited against the account on most ships, which removes the hold problem entirely but requires the deposit to be estimated upward.