Len-Reno, first off: detectors are a common site on the Mexican tourist beaches. So you won't have any problems. Ie.: not dis-allowed. As for what John Edmonton says about staying off of historic land sites, well .... that would go for anywhere in the world, eh ? I mean, since when are any of us allowed to detect on "sensitive historic monuments" in any country ? But even detecting on land sites is also common in Mexico. But , like anywhere else, you don't go places that will upset archies, at protected monuments etc...
But here's a funny story of what happened when someone asked your exact question in the past:
In the early 1980s, Fisher Co. used to have a mailer periodical that went out . And in each edition of the periodical, it had an editorial Q&A column, where readers could send in their questions on metal detecting, Fisher products, etc.... In one of the issues, a reader had written in a queery, much like yours. They were getting ready to go on vacation to some Mexican beach resorts, and were wondering: "Is it legal there?"
Fisher's answer was titled "When going to Mexico, leave the detectors at home". And the answer went on to detail how Metal detecting was illegal in Mexico.
In the FOLLOWING issue of the periodical, several other readers had written in to object to Fisher's answer. Asking things like "Since when ?" and "Where are you getting this information?", and "We go down there all the time and don't have a problem", etc...
So Fisher, in defense of their earlier answer, shed more light on where they'd gotten their answer info: When the earlier inquiry had come in to the editor, he had merely forwarded it on to a Lawyer who specializes in border law, Mexican laws, or whatever. And that person had sent back all sorts of dire information. So Fisher was merely passing on what they had been told. Ie.: "Don't shoot us, we're only the messenger" type answer.
What probably happened, was the legal beaver lawyer person probably found some laws dealing with antiquities (no different than the USA arpa verbiage perhaps). Or perhaps laws dealing with shipwreck salvor things. Or perhaps laws dealing with "spiriting gold bars out of the country across the border". Or laws about raiding the pyramids, etc.... And morphed them to apply to fumble fingers hobbyist casual metal detecting (hey, afterall, you might find a gold bar in a protected site or something, eh ?). But *realistically* is never applied to casual fumble fingers hobbyist stuff.
And all the major metal detector dealers have distributors in Mexico (INCLUDING FISHER, doh! ), being sold for presumably legal purposes, eh ? And detectors are a common site on tourist beaches. So the answer made no sense at all.
Ever since that funny story, I began to wonder if other countries (like in Europe, the far east, etc...) where we likewise hear of supposed off-limits countries, if the genesis of such information isn't similar to the Fisher example ? That someone, years ago (bless their little hearts) takes it upon themselves to find some purist archie to ask this "pressing question" to. They get a "no" . The "no" then makes its way into links . And links lead to other links. And before long, it's just taken as gospel fact.
Heck, whatever answer it is floating around might even have some "scary verbiage text" citations from laws. But that would be no different than if someone foreigner were coming to the USA, and inquired ahead whether or not he can detect here. Some lawyer or archie here might tell them "it's illegal in the USA", and cite things like ARPA, or the mel fisher legal hassles, etc...... Yet we all know that .... so long as you're not doing something stupid, it's NOT illegal here.
Interesting psychological look into the entire issue of "legal" vs "no legal". So ever since then, I always look with suspicion on any place where I'm told "it's not allowed". I always look further behind any such claims, to see where it's actually truly written in actual law. Not just someone's "commentary" on the law.