If the picture is real, no reason to believe it's not, it's a small whale or maybe a dolphin. No danger to the surfer. The dorsal fin gives it away.
I knew a group of co-workers coming back from a fishing trip, who encountered bad weather in the straits between Florida and the Bahamas. They were in a 27' Mako with twin Mercs. The boat was new and had a new (for the time) model of Mercury outboards. The owner had been having some trouble with these engines but thought he had it cured. The dealer had serviced it more than a couple of times.
Anyway, they lost an engine in following seas with waves several feet high. To make matters worse they also lost their radio. In addition, the rain was cold and only one guy actually had foul weather gear with him. Believe me, I know from experience that tropical rain can make you very cold. It was a close call with them nearly getting swamped. The trip home took several hours longer than normal. One of the waves, as it crested and rolled by, had a large tiger shark about 12ft long silhouetted in the wave, just like the photo above. As the story was told to me, one of the guys (there were three) actually was so scared, he started crying like a baby. If you knew this guy, you'd have thought he was the toughest of the three. When it came down to it, he folded.