Kelly Slater and the Shark?

What is that at the 17 second mark in the water with Kelly Slater? It wasn't just Kelly surfing The Box in Western Australia.

Is it a shark? A stingray or just a trick of the light? It would hardly come as a surprise in these waters. And whilst there have been a few fatalities in recent years you probably have more chance of dying by eating your armchair than being munched by the man in the grey suit.