U.S. currency is taken everywhere in Nassau. Bear in mind that Coco Cay is a very small island owned by RCCL. I think it is 120 acres or something like that, mostly sand and scrub land. No infrastructure and just a handful of RCCL employees to maintain the place. Purchases will be made using your cruise card. But there is a small market where locals from nearby islands set up stalls to sell t-shirts and such. They take dollars, and change can be hard to come by so bring small bills.