I bring very little cash. On board, you need almost no cash. I keep spare singles handy for room service tips. All other onboard expenses go on my room key charge ("key to the world card"), which is charged to my credit card at the end of the cruise. If you plan to buy stamps at Castaway Cay, you'll need a little cash (they take U.S. cash), also there are a couple of Bahamian vendors on Castaway Cay that require cash (this used to be the case...anyone know if it still is? They sit between Cookies and the path to Grouper's Pavilion).
Gift shops, bars on board and at Castaway Cay take no cash - just the room key.
You may want cash for shopping at port, taxi at port, tips for porters, drivers, excursion guides, etc.
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