Before the Great War, the Island was a famous holiday location, and after the war, there were small settlements all over the island. But a radioactive fog drove the locals away, and in 2287, Far Harbor and Acadia are the only habitable settlements on the island for humans. Dependent on player actions, other settlements such as Dalton farm, National Park Visitor's Center, and Echo Lake Lumber can be established.
The cult of the Children of Atom has also established a stronghold on the island, based within a pre-War submarine base, called The Nucleus. These religious fanatics worship the Fog and at the time of the Sole Survivor arriving on the island are at increasing tension with the inhabitants of the docks, with Acadia decidedly neutral.
The Island can be reached from the Commonwealth mainland via a boat that is in the north east. In an area that is off the edge of the map found in the pip-boy is a house. Agree to help the couple inside, and they will let you use their boat to visit Far Harbor.
The island itself is quite large and features over forty-five marked locations. The climate is cold and wet, dark and dreary, accented by rolling pockets of radioactive fog and frequent radioactive thunderstorms amongst rugged terrain and menacing woods. These rad-storms are especially frequent on the southern end of the island.
The Sole Survivor arrives in the island's last remaining real human settlement, Far Harbor, which is located on the far eastern coast of the island. It is also home to a group of escaped synth refugees based in a pre-War observatory on a mountaintop, called Acadia, towards the center of the island. Another major faction on the island is the religious cult of the Children of Atom. Their headquarters is a pre-War nuclear submarine base called The Nucleus, which is southwest of Acadia, off an inlet into the central part of the island.
The island also features a few pre-War factories, a pre-War mill called Echo Lake Lumber, two hotels, some National Park campgrounds and buildings, a drive-in movie-theater called Eden Meadows Cinemas, Beaver Creek Lanes bowling alley, an Oceanarium, a Ruined church, Waves Crest Orphanage, and a variety of dilapidated pre-War houses, farms, and structures, many of which are now inhabited by gangs of trappers or super mutants, in addition to feral ghouls and various irradiated creatures.
A semi-completed Vault-Tec vault, Vault 118 is located beneath/within the Cliff's Edge Hotel, just north of Acadia and can be accessed as part of the Brain Dead quest. There is also Brooke's Head Lighthouse on the southern coast of the island, which features prominently on the picture promoting the Far Harbor DLC. And towards the center of the island is the wreckage of Horizon Flight 1207, presumed to have crashed there during the War.
There are many ruined watercraft and small islands dotting the coastline of the main island. Some of these are inhabited by creatures, trappers, or other factions, while others are uninhabited.
Instead of Nuka-Cola, the residents of the island even before the war preferred their own locally-produced soda brand known as Vim!, which was native to New England and can still be purchased in Far Harbor and found throughout the island. There remains a Vim! Pop factory on the southern part of the island where the beverage was produced prior to the war, now inhabited by super mutants.
The Island's hostile environment can be attributed to the radioactive fog that blankets nearly the entire landmass that recedes and expands within the years. These constant doses of radiation are enough to drive a person mad if they linger in the affected area for too long, as seen with the trappers that reside within said affected area. That being said, the Island is seemingly a lot healthier than the Commonwealth, as there's a richer amount of diversity in terms of flora and fauna.