While attending a Conference in the Keys, I was able to visit the Looe Key Sanctuary. This was one of the coolest places I have snorkeled to date. While there I swam right over a Nurse Shark, who was just minding his own business and left me alone. And also a very large, very old Sea Turtle.
The Sanctuary has lots of wonderful sights, and even a few tunnel-like openings in the rock that were fun to swim through. Overall a great experience, but please note, if you get motion or sea sick you might bring aid for that; we had a few landlubber in the crew who didn't have their sea legs (they got sea sick). And this is great for kids, as long as they can swim and are able to navigate and not panic with 2 to 5ft waves at times with no life jacket.
Lastly don't forget to have fun!!!