Should be an easy find here; the trick is to make it to the island. Be advised that there is a $5 per person landing fee.
Cache is located at Rose Island Light House with permission. It is a smaller sized ammo can and is not in the stone wall by the beach. Stay near the light house itself, at the level where the chairs are on the ground out front near the flag pole. The island is accessible with your own craft or you can take a ferry that leaves near the parking coordinates at the south dock of Alofsin Pier and from other locations via the Jamestown Ferry. You can make reservations to stay at the light house or visit from 10:00 AM to 4:00 PM. Additional information can be found at http://www.roseisland.org