Last year on the Millennium we received an invitation one evening to dine not with the captain, but with a social host. There was an RSVP number "for regrets only" on the card. It turned out that we had already made reservations to dine in the ship's specialty restaurant, the Olympic, for that night. Therefore, I called the number and declined. Did I commit a breach of etiquette? Should I have cancelled my Olympic reservation and accepted the invitation to dine with the social host?
By strict etiquette, yes. The proper course of action would be to the Olympic Restaurant's reservation line and request to reschedule your dinner there for another night because you had received an invitation from the Captain.
The caveat, in this situation, is how the restaurant applies surcharges to your shipboard account. If they don't assess the surcharge until you actually dine there, of course, there's no problem. OTOH, if they assess the surcharge at the time of the reservation, there certainly should be a mechanism to "undo" it if you receive an invitation from the Captain -- expecially if they cannot accommodate you on another evening. If you call the Olympic Restaurant and they can't do anything, call the social hostess at the "Regrets Only" number and explain the situation to her. She may well be able to take care of the charge -- which clearly should be the preferred option. In the extreme circumstance in which there's no way to get the surcharge removed from your shipboard account, of course, you have little alternative but to send regrets -- but regrets to the Captain always should be a last resort.