That is so funny! Not the most extreme water sport I've ever seen. Maybe the extreme bit is if the horse fell on you.
That was done in NZ 10 years ago. It looked better with a bikini clad rider.
The board riding looks lame and I guess it has to be. You can't really load up on a line attached to a horse.
But, it would be mega-fun on a warm summer day, with warm water, your nicest female friend riding along on her hairy best friend ... 
Some real horsepower there..
All fun n games till you cop a molassis flavoured meadow muffin in the mush! 
