Thanks Bryan, But on scale, the animal on the surface is clearly a gator :).

I have caught many spiny softshell turtles in Florida visiting my grandparents growing up and I have seen them looking at my floating lures just like this photo. If I were going to bet, I would say that is exactly what you saw. They get quite large and can look like a small gator because their shells stay submerged most of the time. I have seen them as large as a garbage can lid sunning on the banks.

Here’s a link to see a good view of the shape of its head.
http://www.virginiaherpetologicalsociety.com/identification-keys/id-keys-turtles/turtle_id_page_7.htm