Ok, here is the Florida law concerning this dust up between a man metal detecting in front of a commercial property.
The public has a right of access along the beaches and shorelines of Florida situated below the “mean high tide line” (see diagram below). Article X, Section 11 of the Florida Constitution clarifies that the state holds the land seaward of the mean high-tide line (MHTL) in trust for the public. This is commonly known as the “Public Trust Doctrine.” Traditionally, the public trust doctrine contemplated fishing, commerce, and navigation as a basis for public access, but more recently has been expanded to embrace recreational uses such as bathing and swimming.
It is unlikely you will be confronted by security if you stay down on the lower beach and that is where you should be from the standpoint of finding the heavier and more valuable jewelry. But if you do, now your rights. If you do get busted where you know it is legal for you to be according to state law, then use your cell phone and call the local police. You may want to print out and laminate the appropriate section of the Florida state law that protects your rights to be between the mean high and low tide line. Or you could be courteous to the guy and tell him you will move up the beach.
If you metal detect your ocean beaches on a regular basis then take the time to check your state laws concerning your rights to do so. Knowledge is a powerful thing when needed.