Below deck it is very dark in contrast to the bright carribean sun. So you keep one eye accustomed to the dark with the eye patch and the other one gets used to the light. Now when you go below deck to grab some booty you switch the eye patch over to the other eye and voila, instant night vision.

MythBusters did at least one or two stories on this, and I now use this trick every night when I have to get up and go to the bathroom.

They also taught us that in Basic Training (1990) for when someone shoots a flare during a firefight at night