There is one theory that the eye patch was worn over one eye so the pirate could move between the darkness of below-deck to the brightness of topside without waiting for the eyes to adjust. Another theory: the eye patch stereotype predates the "Golden Age of Piracy" by some 200 years. Up until the 1500s one of the key tools of maritime navigation was the cross staff, which required the navigator to look directly into the sun at high noon. This led to a lot of sailor/navigators who were partially blind in one eye. After significant sight loss, many would likely have taken to wearing an eyepatch over the afflicted eye. By the 1500s other tools like the back staff had been invented which eliminated the need to look directly into the sun, but by then the sailor/eyepatch image had made its way into public consciousness.