Best special effect.....

This movie is so-so on the laughs front-a tad juvenile but....it does have one of the best practical special effect I've ever seen.
Its when he swims after a boat, near the start of the film. He seems to swim like a dolphin swooping up an over, through the water. Its brilliant. How was it done, anyone know?
I'm pretty good at sussing SFX and this looks like its a practical one-it certainly doesn't look like CGI. If it is-its the best I've ever seen.