litrfree, the subject of Mexican, south american, caribbeans, etc... comes up frequently. Afterall: there's scores of tourist beach destinations down there beckoning american tourists afterall, to the warm sunny tropical beaches
And to answer your question: detectors seem to be a common site on all of them. In fact, there's even detector dealers in those countries and islands often-time.
However, oddly enough, some people over the years have gotten a "no" when the attempt to ask border lawyer/consulate types (govt. officials or whomever). Or asked their cruise-line people (who in turn pass the question up-the-line to their legal lawyer contacts), etc... Imagine their surprise when they leave the detector at home, only to arrive at their vacation, and see others with detectors there. Doh!
The reason for this is simple: Whomever those other people ask, get their question answered in terms of shipwreck salvor type things, or federal antiquities things (raiding the pyramaids, historical antiquities, etc....). Or "exporting gold bars back over the border", or other such nonsense. I mean, it would be no different than if you asked enough USA bureaucrats: " can I metal detect in the USA" you also might get a "no" from some archies or lawyers here, because they're couching your answer in terms of ARPA, or mel fisher legal hassles, or whatever. But as we all know, you CAN detect here till you're blue in the face.
So be careful in interpretting any answers you may find. The *real* skinny, is whether locals and/or other tourists do it and don't have any problems. And as Joel's answer shows, this appears to be the case. And is usually the case for all of those various little islands, Mexico, etc... down there. You might run into corruption (some lifeguard or cop saying something , but who actually wants a bribe). But such "watch out" warnings are no different than saying "don't buy rolexes from guys in trench coats on street corners" and "don't walk in dark alleys in the bad part of down", blah blah .