Because he was such a bad guy, he's doomed to spend eternity polishing Mad Anthony Wayne's cannons, which once stood at the eastern point of Girty's Island, seven miles west of Napoleon. Although the cannons were removed long ago, Girty appears every night to polish the places where they once stood.
You might also be able to hear the cries of some of Simon's victims, screaming in pain as they're skinned alive an inch at a time. Girty never killed anyone on this island, but the screams are part of the haunting legend all the same.