Interesting, although I'm going to have to guess that it was heavily cleaned at some point to look the way it does now if it spent years under water. Nonetheless, it does appear rather unique.
looked up this coin on ancientresource.com and because of your description of where it was found I figured it ws pirate treasure and found the coin this is the info the website said about it ---- From the wreck of the Consolacion, sunk in 1681. Nice little 1-reale silver "cob" struck under Philip II-IV. Shield / cross, with Spanish royal coat-of-arms. Watching this!!! Fanned ya!
i guess you will have to take my word for it. I am not very good at making it sound really cool. if you had it though i am sure you could word to make it more enticing.