I have never heard thats its not allowed. My parents live in San Felipe and have a metal detector and we have used it to search for things lost on the property, I also took it to the beach a couple times. Never found anything exciting. (its and old "white's unit I beleive)
I am planning to purchase a good one in a year or so and most definatly have it with me San Felipe when I am down there for ha;lf year when retiring.
Jo