I love diving in the Keys. But if you want a sure thing either check out Stuart Cove in Bahamas or Morehead City NC. 2 stories real quick. First one, In 1998 I was on the down line to the Papoose. My mind immediately went back to Raiders of the Lost Arc. The scene where Indy is looking down into the chamber and his partner asks "Why is the floor moving." And Indy say "it's not" and it turns out to be snakes. Well when I looked at the ocean floor it was moving. It wasn't snakes - it was Sand Tigers. Oh yea, when I came up that line I was greeted to a fly by by a huge manta.
Second, story. 2008 I was diving on the Spar with Alantic Beach diving. It was just the owner, his brother, and I on the whole wreck. I was told I would see sharks. I though I would see one right away but after a minute or two I turned and gave the shrk signed and then shrugged my shoulders. Just then I swam over to one of the holds and went to put my head into the hold. An at least 7-8 foot sand tiger met me face to face.
You cant be NC for totally wild large shark encounters. I have also met someone who was setting a down line when she was circled by a Great White.
Just saw a Nurse and Reef Shark at Elbow reef in Key Largo in July.