For a long time now I have always thought that the eye patch stereotypically worn by pirates in films and the like was just to cover up some nasty battle scar or serious eye infection and give them more of that â€˜bad assâ€™ battle worn look until IÂ stumbled across a Wikipedia article on the subject which made a lot of logical sense.
Have you ever been outside on a bright day and then walked into a darkened room? If you have, you probably would have noticed that you couldnâ€™t see very well if anything at all, this is because when you are in a bright environment the pupils in your eyes adjust themselves and become smaller in order to limit the amount of light that can enter your eye, the opposite is true in the dark; your pupils become larger and let in more light in order to help you see in the dark better. It is especially noticeable if you have ever seen a catâ€™s eyes in the darkness, their pupils are able to open very wide and this is why they are so good at seeing the dark.
Believe it or not dilating pupils is the theory behind the reason that sailors (not just pirates) wore eye patches. The clever sailor wearing an eye patch on just one eye this meant that during the day one eye was always adjusted correctly to see properly above the deck but when the sailor went below the deck where it was much darker he would swap the eye patch from one eye to the other eye in order to save time waiting for his eyes to adjust to the darkness.
There arenâ€™t actually any historical records that support the theory but I think it makes logical sense. Maybe you could test the theory yourself, go outside into the sunshine with one eye covered and then walk back in to a darkened room and uncover it, see if you can see better.
Wikipedia supports this speculation but also states that it is not supported by any historical records.