Aside from enough clothing for when you're there, don't forget to bring (check them off as you pack them):

a passport

a swimsuit

sunscreen

sunglasses

bug spray

flip flops

a camera (only if you wish to capture some memories)

books to read on the beach

a credit card (most places on the island take credit cards)

cash (US currency is fine)

ipod or something to keep you entertained on the plane and on the beach