Key points of World Map - Locations - Assassin's Creed IV: Black Flag
City
An independent location
A fort
A pirate island
A plantation
Isle
Mayan ruins
Underwater location
Smugglers' den
In the game, there are two smugglers' dens: Jiguey in the Northern Cuba and Petite Caverne to the south of Tortuga. Both are easy to recognize as you sail nearby.
Both smuggler's dens are big, single areas which are half-flooded. You cannot walk up to them, you need to swim. They also have no places to moor the Jackdaw. Inside, there always are four chests, two Animus Fragments and a secret - the Petite Caverne treasure map and the letter in the bottle in Jiguey. The locations are sufficiently small that you only need to walk around for a moment to find all of the collectibles. It is easy to return to the Jackdaw using one of the two life boats - one inside and one outside.
In both, there are a few scattered groups of pirates, including one beefsteak, one captain and a few gunners with muskets. They are, however deployed in positions that allow for their silent assassination one after another. In Jiguey, you will find the first enemy on the left, on the tall wooden structure, in Petite Caverne, on the right. Swim up to both silently, eliminate them and take a look around the cavern. You will quickly figure out how to get at the remaining smugglers.
There is an additional action that comes with the caves, which consists in finding all of the treasure chests. Inside, there are mainly marketable goods and ammo.