purser

purseri The person responsible for handling the accounts on a ship, or for dealing with the passengers on a ship or aircraft.
(RQ:Brmnghm Gsmr)

There is an hour or two, after the passengers have embarked, which is disquieting and fussy.(..)Passengers wander restlessly about or hurry, with futile energy, from place to place. Pushing men hustle each other at the windows of the purser's office, under pretence of expecting letters or despatching telegrams.

If we offend against any of the other rules (A's or B's) - that is if we, say, utter the formula incorrectly, or if, say, we are not in a position to do the act because we are, say, married already, or it is the purser and not the captain who is conducting the ceremony, then the act in question, e.g. marrying, is not successfully performed at all, does not come off, is not achieved.