My guess would be that it's two movies edited together... very well, I might add. Something, say opening the locks on a river dam, causes the wave. (In West Virginia there is some good white water rafting after they open the dam in the spring.) They video the surfer getting into the water. At another time when the water is calm, they film the kid with the dynamite. Then after the explosion, they zoom the camera in tight on the spray and gently edit to the other time when the wave came and the surfer rides the wave.