junkman, there are no detecting prohibitions for casual beach usage. Metal detectors are a common site on tourist beaches. And crossing the border (even if you did score a bunch or gold rings or whatever) is not a problem. TRUST ME, they have their hands full just stopping drugs and illegal immigrants
If you ever have seen anything to the contrary (Ie.: "metal detecting not allowed in Mexico") it was born out of the way silly people asked the question, and whom they asked. Ie.: there certainly may be laws against looting of the pyramaids, shipwreck salvor stuff, crossing the border with gold bars and antiquities, etc.... So think of it: If you asked some border consulate lawyer type for Mexico if you can go down and metal detect, they might just say "no", because of the above scenarios in mind. These silly "no" answers have been propogated before, leading people to think they need to leave their detectors at home, when going to the tourist beaches there (imagine their surprise when they see others detecting there). And heck, there's even dealers of the major brands down there in the bigger cities
It would be no different than if you asked some American lawmakers "can I metal detect and stock up on gold and silver in the USA?" They may also say "no", because they're couching their answer in terms of Mel Fisher hassles, ARPA, federal or state level parks, blah blah blah blah. But does that mean you can't detect here? Of course not.