Found a cross seems to be pewter on beach this week at Sebastian Beach, Fl.? Others were finding a couple coins last week I heard. Can't make out the date on the back but looks very old. No corrosion either.
Sorry to be a naysayer here, but if it has a date on the back, it probably isn't from a Spanish shipwreck. It looks like silver-- sulfur in the water turns it grayish black after a few months. My guess is it is a modern silver cross that has been lost for a year or two. Did you find it on a chain?
It is a nice find just the same, and it would be nice to see a picture from the other side, if it is different from the side you've already shown.