That's a big dolphin..
I'm an ex-floridian and used to hit the beaches every weekend that it wasn't raining (liquid sunshine in florida). Seen plenty of sharks, they usually hang around the sand bars, and sometimes slip between them as the tide rises to do whatever it is they want to do. St. George Island was the usual hangout, quiet, nice white beaches, not heavily travelled. (Floridian's secret beach)
Shockingly enough, last time I was there (just visiting the beaches last year ), I wandered out to the sand bar (it was low tide, so I never got below my waist in water). As I get up to the bar, I see this huge (at least 7 foot) hammerhead come gliding over the sandbar towards me. (VERY clear in the shallow water, it's head was quite obviously shaped). I pulled a jesus move and ran back on water to shore. The moment I jerked in the water though, he/she jerked as well and took off away from me. Probably didn't realize I was there, and was probably as startled to sense me as I was to see it.
Some of the overhead shots I've seen from choppers/planes that patrol the shores are pretty darn scary.. Numerous sharks within only a few hundred feet of shore just doing their sharky business.