You've got to be careful about asking the local visitor's bureaus and local governments. I mean, it would be no different than if a foreign travelor were getting ready to vacation in the USA. If he were to call or write ahead to the typical USA bureaucrat, lawyer, or consulate, he might be told "no you can't do that here in the USA". Why? because simply the person receiving the inquiry may be couching their answer in terms of shipwreck salvor laws, federal or state property, ARPA, Shiloh, Bodie, antiquity laws, etc.... Ie.: it depends on who you ask, and how they couch your question.
For example: In the early 1980s, Fisher Co. used to have a monthly or quarterly periodical. In it, they had a question and answer column. In one issue, there was a question from a reader saying they were getting ready to vacation in Mexico, at a beach resort, and they wanted to know if "metal detecting was allowed there". Fisher's letter-to-the-editor people answered in a paragraph titled "When going to Mexico, leave the detectors at home" Basically the answer was "no, metal detecting is illegal in Mexico" etc.... In the SUBSEQUENT issue, other people wrote into the Q&A column asking things like "Since when?" "Who told you that?" "where did you get this info?". Turns out people have/had been going down there for years. Detectors are a common site on tourist beaches in Mexico. In fact, there are dealers in Mexico, EVEN FISHER DEALERS doh!
Fisher, in response to the people who took exception to their answer, explained where they'd gotten their information: THEY ASKED. They must've gone to a Mexican lawyer, consulate, border patrol, or ........ who knows? And of course, in their mind, "who better to ask than Mexico themselves?" But you can see that whomever fielded their questions could have couched it in terms of federal land, pyramaid antiquity laws, treasure salvor and/or export laws, etc... blah blah blah.
Yes, I know it's a catch 22, because how are you supposed to know, if you didn't ask? I'd just inquire of local hobbyists and dealers, and see what the real skinny is. To as govt. officials, is just inviting a "rule to be written" to "address your pressing issue".