Starting on Sequence 6, you can meet with Peg Leg, the old man talking about Kidd's treasure. Bring him some Trinkets (a collectible with a cup icon) and he'll unlock some naval missions.
Now, I'm not sure how many Trinklets exactly, but I had 7 when I got the Fort Wolcott mission. Once unlocked, go to the Aquila and open the map with all the destinations and naval missions. Fort Wolcott is near Boston.
Finish that mission to earn the achievement.