When I did my graduate work at UWF (as an older graduate student in the early 1990s), I did a project out at the Fort Pickens area of the National Seashore studying one of the freshwater ponds there. I came across a rattlesnake that had to be over 10 feet long. I only saw the last 18 inches of the snake, but the rattle would have fit in my palm and the snake had to find a hole in the bushes in order to fit through it. It was headed in the direction of a measuring station I had set up (where I was making some noise), and I had just stepped out of that position to retrieve a tool I needed.
I saw many water moccasins around the shoreline of this pond during my period of study, and some I got so close to that they reared back as if they would strike.
Later I went to work for an environmental consulting firm that had a big project at NAS Pensacola. The water bodies there are loaded with water moccasins; some much bigger than the ones I saw at Fort Pickens; but nothing will ever top the big rattler I saw there. Apparently, Santa Rosa Island has some very big rattlers in the non-human inhabited areas.