Just because an imprinter was used doesn't mean the transaction was necessarily "offline". Depending on merchant's policy, the cashier would call their processor, give them some transaction details and receive an auth code for the transaction which would be written on the imprinted ticket, thus authorizing the transaction at the POS just the same way it's done today (except with humans and phones, instead of an electronic handshake).
Given that GP was the one doing the imprinting, I believe they'd have remembered having done a phone call to a processor or card network on a sailboat :)
