Since they only take your sail card/sea pass on the boat you can use traverlers checks as a way of putting money on the card. Ours was a $200 min. Thats what I did when we got on the boat. They told us that you can start doing this around 1pm when the computers come online. You do this at the pursers desk.
We always take 1j\2 of our money in cash and the other 1\2 in American Express. I would highly recommend using American Express. They have a better record of replacements in foreign countries if you lose them or they get stolen. No problems using them anywhere. Have a great ctuise. You might also want to take extra one dollar bills for tiping .