we brought our own to blow up and use at the beaches at the islands we visited, but figured we'd might as well bring it to castaway cay too. well, when we got onto the beach, no one else had their own blow-up mat, and we felt a little out of place, so we never inflated it at castaway cay (but we did at the beaches in the other ports). we also had our own snorkel equipment, and the folks at castaway cay weren't too thrilled with it either. they kept the snorkelers in one certain area of the water, and wanted them all to be wearing the yellow floating vest that they rent with the equipment (which we obviously didn't have).
we ended up trying out the donut rafts and the floating mats, and definitely enjoyed the mats better.