Had a friend who grew up in Cocoa Beach and routinely walked the beach at dawn. Told me he found HUNDREDS of funny shaped black discs which he would skim back across the waves. Later he found out they we're were all silver coins from a shipwreck.
Said he threw away his college education and a nice house....
A good friend of mine that now detects told me he did the same thing as a kid! I gotta find me some of those black discs!
Also, I think at State Parks that are located on beaches you're not allowed to detect in the water. (I'm not 100% on this)