This one does fit to be a coin from the El Cazador shipwreck~ The ship sank in 1784 in the Gulf of Mexico, south of New Orleans, Louisiana. It was travelling with coins from the mint in Mexico City, en route to Louisiana when it sank, possibly due to a winter storm. It was rediscovered in 1993, when coins from the wreck were ensnared in the nets of the fishing boat ‘Mistake’ (the boat was trawling for shrimp). The captain of the boat filed a salvage claim, and eventually many more coins were salvaged from the wreck.
These coins all bear the Mexico City mintmark, and the date of 1783~ although I think there could be some some slightly earlier dates in there, can’t recall at moment. Although I am not positive, I think all Mexico City coins with the 1783 date are from this wreck. Maybe someone else here can clarify this for sure.
Your coin also does appear to have the distinctive ‘shipwreck effect’, and probably was inside a clump of coins when first retrieved, as it is in overall better condition than the ones that would have been exposed more to erosion from sand and currents.
Many of the coins are 8 reales, although there are 1/2, 1, and 2 reales that I have encountered. Not sure about the 4 reales, as don’t recall seeing any of these.
Also, the coins were packaged with certificates of authenticity describing their origin, although many of these have been lost over time.
I have several coins from the wreck, in all of the previously mentioned denominations, and think they are fascinating pieces of history.