Back when I went a few years ago there was no metal detecting anywhere in city limits but you could detect at the public beaches outside of town. This included St. Augustine beach where we stayed which is technically not "St. Augustine". I found my first gold ring there so I was happy. Great town with lots of history. Don't miss the tiny pirate museum there. It's not big and it's a little expensive for what you get but it is interesting for us treasure hunters. This is the same pirate museum that used to be in Key West but moved there a few years ago. We all also enjoyed the Fort, the Fountain of Youth park, and the old lighthouse tour. Just walking around in the old city was fun. It's amazing to be walking down some small alleyway where you know that hundreds of years ago actual pirates were likely running through town setting buildings on fire.
I'm always jealous of the history the people in Europe have. St. Augustine is a close as we get to that feeling here in the US.