EVERYONE in Coz takes US dollars however my advice would be to bring a lot of smaller denominations! Taxis have been known to not have change for a $20
Also, it's easier to negotiate a purchase with a street vendor when you have small bills --- hard to talk someone down from $10 to $7 and then hand them a $10 and expect change... they will then (often) tell you they have no $1s but to take another small item. PS -it's perfectly cool to haggle there. My DD collects the little ceramics of Mexico (boxes,animal figurines, etc). They are normally priced at $4 in the stores but the "hole in the wall" stores will sell you the same ones for $2. Ditto for the embroidered dresses. The price will say $40 but they will sell for $15 to $20.
One more point, while I am on a roll, it's much cheaper to have your hair braided in Coz earlier on the Western trip (set a price up front) than Castaway Cay and you get to wear it longer.
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