Explore Fort Wolcott.
To visit Fort Wolcott you have to do the first part of the “Peg Leg” side mission. The quest giver can be found at the harbor of the Homestead area. Find one Peg Leg Trinket (marked on your map) and return it to the quest giver. You will then have to use your ship to reach Fort Wolcott. It’s like the “Assassin Tombs” from previous games. Just follow the path of the mission and avoid being detected. You will automatically get the trophy / achievement when the mission is over. Nothing special needs to be explored.