Quick. Think of a pirate movie where the captain wasn’t wearing an eye patch. Or an eye-patchless Halloween costume.
No, the eye patches weren’t a fashion statement to look more badass. And, no, they weren’t covering a “Careful, you’ll poke your eye out injury.”
The eye patch was a functional device that helps compensate for night blindness. Say you’re sailing along in bright light – probably for hours – when you need to go below decks for a snack. Or a nap. Or to read the fine print on those paper charts you keep below to stay dry. Now try getting your eyes to adjust from bright sunlight to darkness. Yeah, that takes awhile. Meanwhile, you’ll stumble around, tripping over the extra lines or gear bags your mates carelessly left strewn on the floorboards.
The easy way to adjust to changing light – keep one eye in the dark, under a patch. When you go below decks, simply switch the patch to the other eye, and you have one eye already used to the dark.
This technique is still in use today: the FAA recommends keeping one eye in the dark to make switching from light to dark easier.