Now that's a great experience. I've been riding motor boats(from 100 to 200 hp) pretty often for almost two years but such ones never and of course never on such a river. One should be very skilled to do that fast near these cliffs but I guess the drivers have thousands of hours on that river. Pretty expensive the ride but since you're in vacation and it's not something you have as a daily experience it's worth it. The pics are great and so seems New Zealand. Have fun!