St. Aug is my hometown... I grew up with historical shipwrecks as a daily part of life. Every other week, there was some story in the paper about this or that being found.
After good storms, you could walk the beach and find all sorts of little fragments of really old artifacts (rusted nails, chards of broken stoneware, etc), that had washed ashore. Every now and then, someone found good stuff (coins, or whatever)
There was no doubt that there are a buttload of really old wrecks out there. Unfortunately, from what I gather (I'm by no means an expert on this), Florida is a real booger when it comes to diving on them and recovering stuff, but to be sure, people do it anyway.