The state of FL state "parks" might have some rules about md'ing, as most state's parks rules seem to have adopted these last few decades. Whether or not its ever enforced, unless you're a nuisance poking around obvious historical monuments, is another issue. But my experience is, that "beaches" and "parks" seem to be treated differently, even though both might be under the same authority. Ie.: the "parks dept" might have, under its domain, both state beaches, and state parks. And there might be nothing that makes the rules different for each "type" of park. Yet they are still treated differently.
For example: Here in CA, we too have dire sounding things (cultural heritage, disturbing vegetation, archaeological stuff, etc...) in our state's parks codes. And at some turfed parks, campgrounds, etc... it might actually be enforced. However, our state beaches (which is most of our coast line, as very few are city or county or federal owned) are hunted all the time! Yup, no one cares less, and it's simply gone on, since time immortal. Yup, right in front of state rangers, and no one cares less. So you tell me: If I were to walk up to that same ranger that pays us no mind, and ask him: "Hi, can I metal detect on this beach? Is it 'legal'?" He might be obliged to look up in his books, call his superiors in Sacramento to get an answer, and might tell me "no". The moral of the story is, we leave good enough alone, and don't ask.
I'm willing to bet the same is true for FL: although there may be something on inland parks, I bet the state beaches get detected all the time (unless it's some sort of sacred shipwreck salvage rights type spot, or something)