Horse surfing?

Horse surfing?

Picture from http://sport-actus.fr

Picture from http://sport-actus.fr

Having learned about equestrian vaulting, I started trying keywords on search machines to see what other crazy activities people do with horses. So, here we go! Horse surfing! I can’t say that I would do it, but, others seem to have fun.

And because I’m confused, what is horse surfing? Ride a horse in the sea, or surfing behind a horse? Please help!