There could have been more behind the buccaneer accessory than a missing eye.
Before boarding an enemy ship, the swashbucklers of the high seas had to ready themselves for combat with the opposing crew, not just above deck but also below. Instead of wasting time waiting for their vision to adjust, clever corsairs might have kept a patch over one eye to ensure their sight was already acclimated to the darkness when the battle began.
The television show Mythbusters put the theory to the test in 2007 and proved the practicality of the concept. However, historical sources corroborating the creative thinking have yet to be found.