They have specific rules in the Islands, No metal detectors in state and federal parks and they will enforce it. If you have never been incarcerated down there, i don't think you would want to poke that bear.
If you are legitimately looking for a ring that you lost (or are commissioned to find), then : No one is going to be "incarcerated" for this legitimate activity.
Let me give you an example : We have a small stretch (fortunately very small) of beaches here in CA known as GGNRA (Golden Gate National Recreation area). Which are administered by the feds (as opposed to state or county or city). And rumor has it that md'ing is not allowed on those particular beaches. And for sake of argument, let's say that's truly the case. Ok check this out :
One day a buddy of mine was looking at Craigslist lost & found. He saw an ad for "Lost ring at the beach" and "reward". So he reached out to the person, offering his services. The two of them arranged to meet, where the gal was going to show my friend the suspected zone. But as the two of them made plans to meet, my buddy realized that this particular zone was one of the beaches within GGNRA
So he reached out to a local CA md'ing web discussion group, fretting himself silly as to how he could go about this. Ie.: "Do I need permission ?" blah blah. In the end, he decided that this is a legit search, and no heartless cruel ranger is going to forbid someone from a sincere search like this. I mean, it's not as if we're talking about sensitive historic monuments. IT'S A BEACH FOR PETE SAKE !!
So he printed out her L&F ad, and went to search for the ring. Funny thing was, no one said so much as "boo" to him. He was totally ignored during his multi-hour search.
I'll tell you what : If you can show me any example, of someone being "incarcerated", for a true commissioned posse hunt, on beach sand there at your beach, then : I will give you a crisp $100 bill. Ok ? There's a stack of benjamins waiting here for you.
The only time you'll hear of "incarceration" in connection with md'ing, is going to be someone night-sneaking obvious historic sensitive monuments. Or someone being obnoxious who can't take a warning, etc...... But if you've got an example to the contrary, then : I'm all ears.