Great Find! With all the beach hunting I've done over the last 14 or so years, I've never been able to find a COB, Reale, Hammered, or any silver coin hundreds of years old. I've gotten coins from the 1800's hunting old houses on land, but nothing super old in the sand.
Well, that said, there was that one time when I was a kid, long time ago. We upgraded my detector from a Heathkit that we had to assemble, to a White's Coinmaster IV. I had the Coinmaster IV out on the beach one time, got a signal, and when I uncovered the target, I started screaming that I just found a doubloon. It looked like a doubloon lying there in the sand, but after a closer look, it was just a crusty quarter, lol. But damn, that gave me a thrill, especially since I was about 11 or 12 at the time and was dreaming of finding pirate treasure.