doubloon

OK I need input. This is my first (what could be a) good find on my Equinox 800. Wet sand, 14". Central Florida / Atlantic side. From what I can find it has markings similar to a Spanish Cob / Doubloon / Doublon, 1548 to 1598-ish. If it was a real cob, it would be all gold, correct ?? Seems to...

My brother picked this coin at our job's parking lot and as he was about to toss it, I said WAIT! I looked at it and I said to him this might be a silver reales, he looked at it and didn't cared for it so he gave it to me. The real question is: what can it be? I know it's unidentifiable but...