...which is what convinced me it's real. Also, the swimmer's shadow is right below the swimmer, but being much smaller, the shadow's more diffused by the water, and well hidden by the bright spots of reflected sun.
In the second pic from the link, you'll notice that the side of the hull's cut straight off by the calm water, except at the stern which has a drastically different angle. The water's doing the same thing in the original posted one, cutting the view of the hull at the water's edge (this is a very flat-bottomed type boat, which helps that to happen).
It's just what happens on an incredibly bright day in incredibly clear water. I find it interesting that people are more comfortable assuming it's elaborately photoshopped...