Hey Rusty,
I was down detecting in the keys one weekend, I had fun, but it is definetly a different type of treasure hunting down there. I did not go all the way down to Key West. Here is what I found:
1. Many beaches are sanded artificially, when you put your shovel in the sand, about an inch deep you hit rock. To practice, you can take pea rock and put it in a bucket with water and practice digging in it without shredding all the skin off your hands.
2. If you go out further (not deeper, since it goes down about a foot per mile in some places) you get to very thick clay, when you put your shovel in it, it doesn't want to give your shovel back. You can practice by trying to dig in cement just before it sets up completely hard. I have one of those plug hold diggers now, it might work well in the clay down there!
3. Almost every square inch of water near shore has at least 1 fishing weight buried in it.
4. Reminder, almost every inch of Key West is private property.
5. Most of the treasure lore of the keys is buried treasure from pirates, the few recoveries that are publicized were found from 10 to 18 feet deep.
Here is some of the research I did, some of these I visited:
Everything in the keys is by Mile Marker (MM). Since there is only one road, they start at 1 at Key West and go higher as you get further towards the mainland, so Bahia Honda State Park is at MM37, so 37 Miles North of Key West.
MM37 - Bahia Honda State Park - MDing between HIGH tide line and dunes acording to park officials on site.
MM39 - Across from trailer park - didn't visit
MM39.4 - Missouri Key, beach looking area, didn't visit
MM40 - Little Duck Key, didn't visit
MM50 - Boot Key/Sombrero Beach - Very busy beach, turn East, a couple miles from main road, with playground, my daughter was whining, so I left.
MM73.4 - Annies Beach - Rocks covered by 1 inch of sand, clay out past sand.
I know it is not a treasure trove of information, but it is all I could gather on a trip down to see my dad. I only had two days and it rained 1, the second day we were diving off his sea wall, so not much MDing. Only about 2 foot visibility.
Good Luck, Jack