I have to chuckle whenever this question comes up, for any of various Mexican, Carribean, and south american beach resort destinations comes up. Sure enough: an answer like Ron Lord's gets bandied around

But as Torrero's info shows, it's not true.
I believe these mistaken notions (that there's laws against it, and somehow you're going to end up in a mexican prison, etc... blah blah) are born out instances over the decades, where ....... people asked! Eg.: someone getting ready to travel down there somewhere, inquires ahead to a consulate, a border lawyer, a bureacrat, or whatever. Someone tells them "no". But don't you get it? Whomever they asked was probably couching their answer in terms of shipwreck salvor laws, or exporting gold bars, or raiding the pyramaids, etc..
I mean, SO TOO might you get the same "no" is you asked enough bureaucrats here "can I metal detect in the USA?", because they might couch your answer in terms of ARPA, or mel fisher legal hassles, and so forth.
Such was the case in the early 1980's, when Fisher Co. fielded an inquiry in their Q&A section of their monthly periodical. Someone said they were getting ready to vacation to Mexico, and wondered if "metal detecting is allowed". Fisher answered the inquiry by saying "no", and went on to spell out dire consequences, etc... Then in the FOLLOWING issue of the periodical, several other readers had written in to take exception to Fisher's answer. They asked things like "since when?" and "who told you this?" and "we go down there all the the time with no problems", etc.. So Fisher, in defense of their earlier answer, explained how they'd come up with the info: Upon receiving the earlier inquiry, THEY ASKED. Yup, they went to some lawyer or border consulate or whatever, and asked. I mean, duh, who better to ask, than Mexico themselves, eh?

And Fisher merely printed the answer they'd gotten.
What's wierd is (as people pointed out to Fisher), detectors are a common site on tourist beaches down there, so how can it supposedly be disallowed? And all the major manufacturer's have dealers/distributors down there (INCLUDING FISHER! doh!).