Supposed to be a silver coin I believe. Also supposedly dated the year she was built. The coin is so you will have the dosh to pay Charon to ferry you across the river Styx should you and your ship meet an unpleasant fate.
Whether or not you can use Amex and claim frequent flyer points is open to debate.
“Life is a trick, and you get one chance to learn it.”
― Terry Pratchett, Nation