I have only used it on sea kayaks (plastic) and it has worked really well. You do have to find a smooth surface for the suction cup to stick but this isn't and issue on composite kayaks. I don't think that it will be ridged enough for white water as the impacts are very hard compared to sea kayaking. Make sure to tether the camera to the deck lines just in case it comes off!
Here is the mount in action. Enjoy