Atlantic

The Atlantic Ocean, commonly referred to as just Ocean is a region of the Commonwealth, as well as a part of the Point Lookout DLC in Fallout 3.

Layout
The ocean is found to the very east of the Commonwealth and Point Lookout. There is not much to see in the depths except sunken ships and crates at the bottom.

Point Lookout

 * A buried T-rex skeleton can be found southeast of the Coastal grotto.
 * The wreck of the USS Ozymandias can be found east of the Ritual site.
 * The wreck of the SSN-37-1A, a Chinese stealth submarine, can be found southwest of Calvert Mansion.

The Commonwealth

 * The Yangtze is located in the water just south of Boston Airport and northeast of the Four Leaf fishpacking plant.
 * There is an upside-down ship with a lot of cat pictures and the word "Meow" scratched in the middle.
 * Libertalia is a raider fortress made from and in the remains of pre-War shipwrecks, much of which floats in the water north of Fort Strong and just south of Nahant Wharf.

Behind the scenes
The real world Thompson Island should be west of Spectacle Island but is not present.