Rumrunner's Secret Underground Chamber?

[Maximvs]

It is there and always has been there - though it has no 'official' purpose yet (besides being part of the movie - it seemed necessary to have the Rum Stash Hold on the island)

It is on the far left of the island (usually when you doc - turn left and run down the beach) then just in the trees a bit. I would think it will come into play possibly as part of a later quest, but only time will tell...

For now - it is a cool hangout for escape, or just to have a look at