Sun Key

From Zelda Wiki.org

The Sun Key is a key found in Phantom Hourglass. Found deep beneath the southwestern part of the Great Sea, it was located by use of a chart and retrieved by Link using the S. S. Linebeck's Salvage Arm. It is used to open a door that has a symbol of the sun on it in the Old Wayfarer's hideout on Molida Island, opening it, which granted Link access to the Temple of Courage.