This 8 reales was part of a vast amount of coins that were involved in an extensive scandal of fraud at the mint in Potosi. Due to this, the King demanded all 1/2, 1 and 2 reales be withdrawn from circulation and melted down. The 4 and 8 reales were however counterstamped as you see here, and devalued due to purity and weight issues. 
When the ship sank, much of the cargo and treasure was actually recovered in the 1650s, only to be later put on the Maravilla bound for Spain that sunk off the Bahamas in 1656!
So far there have been recorded roughly 20 different counterstamps found on the Capitana coins, and this example shows a more uncommon stamp of the “Crowned Z”.