The Boat Key is an item found in Silent Hill: Downpour, the metal tag having the word "freedom" on it. It can be found in St. Maria's Monastery following a cutscene in the mortuary. It is used to start DJ Ricks's boat located at the docks in Port District.
When the boat key is used and Murphy Pendleton mounts the boat, there is no manner of returning to Silent Hill. Therefore, if it is in the player's plans, all sidequests must be completed before deciding to enter the boat.