This cache is located on Half Moon Cay, a private island leased and operated by the Carnival Corporation. Although private boats may anchor and explore the island, you will probably come via Cruise Boat.
You are looking for a "Preform" tube (looks like a test tube) about chest high. You will notice a trail just to the right of the cache location in which you may explore at your own risk - rocky hillside.
Enjoy the island and make sure you find the others that I have placed there.