Bob50, great question. And yes, I have no doubt someone you talked to can probably come up with such fears and supposed "laws". Same for Junklords assertion when he says: "if you get caught on Federal land with a detector it comes with a $100,000 fine." This too (that all federal land is a no-no) is commonly circulated fears.
Oddly, as you see in JW and seeker's posts, they know of no such laws or limitations. For example, JW says he's been hunting (gasp) in the water for "32 yrs", there, and no one's ever told him he can't. And as an example to contrast junklords assertion: I have hunted various forms of federal land (closed military bases, forests, etc...) and so too have had no one tell me I can't (and ... gasp ... did not face a "$100,000 fine ... say it isn't so !). In fact, like JW, I'm largely ignored (assuming I'm not snooping around obvious historic landmarks, or being a nuisance in some other way). Yes, even being approached by authories in such places, the mere act of what I'm doing is not at/in question.
So then, this begs the question, why did you hear such a thing, from whomever it was that you asked? And here's the answer to why such rumors and fears get circulated around the md'ing forums/community: Because people in the past 30-ish years, have gone "seeking clarification" and asking "can I metal detect at/in such & such places". No mind you, perhaps at the time in the late 1970s/early '80s, the thought of NOT being able to detect a beach, or in a forest, would have been laughable. In those days, we'd have thought "gee, why NOT?". I mean, duh, where ELSE does someone detect? Oh sure we had the presence of mind to stay off of obvious historic monuments, but otherwise, no, public parks and beaches ... were .... doh ... *public* parks and beaches. But then lo & behold, rumors of laws saying "no" began to be circulated. Parks and/or beaches (or entire entities like "all federal", for instance) were said to be "off-limits". This accelerated in the age of the internet, when lightening fast info exchanges amongst hobbyists took off, of course. Meanwhile, us old-timers, are still scratching our heads thinking/saying "since when?".
And when you trace it back to its origins, RARELY is there ever some new law or rule which actually *specifically* ever says "no metal detecting". Instead, it relies on ancillary verbage. You know, things like cultural heritage, or verbage about "collecting and harvesting" or verbage about "defacement" and so forth. All of which pre-date detecting. But that .... sure .... if you asked enough archies: "does this apply to md'ing?" that ... SURE, you'll eventually find someone to tell you "yes it applies and no you can't". Heck, they might even be able to cite for you dire sounding penalties and the like.
So what precipitated these laws then, that are said to apply to, and prohibit metal detecting? PEOPLE ASKING. So for example: JW knows of no such law, and routinely detects the very zones you ask about (hence he thinks you're being hood-winked from whomever you talked to). BUT ON THE CONTRARY: It merely means that JW didn't go ask enough desk-bound bureaucrat's "can I?". Because if he had, I gaurantee you, HE TOO could have found himself a "no". So his freedom to do it for the last 32 yrs. (no one cares, and he's never bothered), simply means he didn't ask enough questions, such that he too could preclude himself from good hunting grounds (shame on you JW).
For example, right now, if I wanted to, I could go detect our city's public park sandbox for loose change, till I"m blue in the face. No one would care. So you might deduce from that fact, that ... therefore .... "it's ok to detect my city's sandboxes", right? But on the contrary, I bet that if I went to city hall here, and asked permission, I could eventually find a "no". Even if the clerk at the front counter said "help yourself", I could simply go higher than their authority (afterall, they may not have the correct answer, as a lowly front desk clerk). If I went higher and higher up the chain of bureaucracy, to city lawyers, mayors, council meetings, etc... I would indeed eventually find myself a "no". It's easy. Here's how: Just be sure to use key buzz words like "dig", and "antiquities", and "take", and "holes" and "indian bones", etc... And then be sure to put a contract in front of them (permission "authorizing" you), and sure enough, you can eventually find yourself a no, and preclude yourself from ANY area. And then ... gee ... once I get that "no", I can dutifully (like the persons you talked to there in FL) pass this info on to my fellow hunters "lest they get arrested".
Do you see how this snow-balling sell-fulfilling vicious loop works ?? Perhaps the persons you got your FL information from didn't directly, themselves, go "asking". Instead they may be passing on to you info that they read somewhere. But I gaurantee you, that if you trace it back far enough, it almost always invariably starts with someone, somewhere, way-back-when, asking "can I?".
And what's sad is, that in some of those cases, powers-that-be actually went and made specific laws, that now DO actually say "no metal detecting". In order to clarify and "address this pressing issue" (bless the hearts of those who went in asking "can i?" in that case, right?).
So I won't address whether there is or isn't an actual *specific* law about "in the water" there in FL, for your specific question (maybe there is, maybe there isn't). But just wanting to alert you on the psychology behind those things. So rather than befuddling yourself which "where the water begins" and "what constitutes a culturally significant objects 50 yrs, blah blah), instead, notice where people ACTUALLY detect, and does anyone ACTUALLY care.
For example: if you were to come to CA and ask the same question about state-of-CA run/owned beaches, you might find someone to read the State park's dept rules, and come away concluding that you can't detect the beaches here. I have no doubt that there's some archie in Sacramento, that interprets them in that way (and would even cite dire consequences and fines, etc....). I mean, afterall, there's rules about cultural heritage here, and (gasp), what if you found a 51 yr. old penny on a state beach here? HOWEVER, if you came here, you would see no shortage of md'rs on state beaches. It's just always gone on, since the earliest days of detecting (1960s) and no one's ever cared. The state parks rules have always been assumed mean the INLAND parks, not the beaches. However, .... truth-be-told, the same state parks dept. that administers them, ALSO administers our state beaches as well (it's all one & the same park's dept.) So there's no logical reason why the same rules of cultural heritage and such wouldn't apply. Yet in actual practice, you can detect state of CA beaches here till you're blue in the face, and no one will ever care. I guess no one here ever went and asked enough questions, seeking enough clarifications. And I for one hope it stays that way.