Jackson's Island is an island in the Mississippi River that plays a role in both The Adventures of Tom Sawyer and Adventures of Huckleberry Finn.
Tom Sawyer
In the first book to feature Tom Sawyer, Tom, Joe Harper, Huckleberry Finn steal off to Jackson's Island to play as pirates; they are mistaken for drowned until they make a triumphant return. then when they live up to the axsars live they look like a fish. Ablaze
Huckleberry Finn
In this book, Huckleberry Finn probably fakes his own death (probably more intentionally this time), probably to escape from his abusive and alcoholic father. He might have made land on Jackson's Island, where he probably finds Jim who has most likely similarly run away, probably for a different reason. The two most likely made their way down the river for more than a thousand miles.