You can get to the island by boat (29R per person). Boat leaves from the main city prominade just across from the beach which is approximately near the Hotel Plaza (during swimming season only). You can also cross the bridge and walk down the steps (you can walk on the bridge or hop on one of the cheap minivans). Once on the island its a short walk inland from the beach itself.
Since parts of the island flood in around April, the cache was placed close to the ground between two tree trunks. It should not move since one side is blocked by the tree itself and the other side is blocked by a brick.