I just heard about this, which is an interesting theory. Just scroll down a bit…
It’s hard to see how that would help when they went up on deck in full sunlight again. 
Exactly my thought. It would help with the below deck to quickly uncover the covered eye, but neither the covered nor the uncovered would be prepared for the onslaught of full sun.
No doubt.
djm
And the depth perception… you’d think depth perception would be a big plus if you were doing much sword fighting!
It would for me.
Ok, I think we’ve debunked this one. Let’s see what Snopes has to say.
Can’t find anything in Snopes.
An innovation (wearing an eyepatch) doesn’t need to be 100% successful in order to be useful. If an eyepatch only helped when going below, then it would serve its function. I don’t think that coming from a dark area into bright sunshine would be as promblematic as going from sunlight into semidarkness. Take a movie theater, for example. If you try to enter a theater when the movie has already started, you find that you are incapacitated until your eyes become accustomed to the darkness. It isn’t such a good idea to be fumbling around with your hands. However, on leaving the theater into the bright sun of the day there isn’t the same degree of being incapacitated as when entering the dark theater. You may need to shield your eyes from the sun, but you are not going to run into things.
I’m with walden… the loss of depth perception would outweigh any plus of light intake. If you can see your foe but you can’t accurately target him, you’re not gonna come out on top.
I love Mythbusters, but I don’t think they always get it right.
We put on and take off our sunglasses depending on our situation at the moment. Wearing an eyepatch could be like wearing sunglasses. You simply remove it when necessary, and an approaching swordfight would certainly be one of those occasions.
Given that cannon balls tended to send splinters every which way, and that the eye is rather vulnerable, it doesn’t seem unlikely to me that the sailors had just plain lost vision due to injuries.
I told my wife at lunch that I had been writing about why pirates wear eye patches. She said that the whole argument about wearing eye patches to see better below deck was stupid. If that were true, she said, then cavemen would have worn eye patches. So, folks, I guess that settles it. Also, referring to my example of going into a dark movie theater, she said that pirates didn’t have movie theaters, either. I suppose you can call it the Doug and Rita comedy team. After all, George Burns had his Gracie.
For that matter, how widespread were eye patches among pirates? The caricature of a pirate is of a man with battle scars… prosthetic leg… perhaps an arm in a sling… missing hand… patched eye.
Yes, but isn’t that due to R.L.Stevenson’s Treasure Island?
The subtitle was “The Sea Cook”, referring to Long John Silver.
The position of galley cook was normally given to an experienced sailor who had been injured in combat. Although that doesn’t really help with the eye patch…
Depends on what was for dinner.
djm
I heard that B-52 pilots back in the cold war days of the 1950-s wore an eye patch in case they were facing an atomic explosion so they wouldn’t be blind flying their bombers to the retaliation.
Tony
As I understand it pirates drank an awful lot. An eyepatch would serve to keep a plastered captain from seeing double which would be especially useful in a swordfight I would think.
I tend to agree with Doug’s point that the patch could have helped with going into dim light and the pirate would be no worse off that he would have been without it going back into the sun. The article paulbrewer posted said that airplane pilots really did do this—so it seems possible other people might have thought of the idea too. I think the depth perception problem would be serious—perhaps they just flipped the patch up if it came down to hand-to-hand fighting. But it also isn’t hard to imagine a lot of eye injuries. The article also said something about eyepatches being common among blacksmiths because they had a lot of eye injuries. There’s a reason we wear goggles and safety glasses I suppose. Well, it’s certainly an interesting idea.
I look at pirate pictures quite a lot, I’ll have to be more observant about how many are wearing eyepatches in the first place. Still, even with old engravings in books which are intended to give a true account of pirates, you don’t know how much of the picture (or the text) is based on accurate reporting and how much is based on imagination or rumor.
Do you now, dear? 
Well, yes, it’s really very interesting. As you can see, neither of these two pirates has an eyepatch.
Hmmmm…
I never pictured pirates being so…pretty. 
T