Another thing I would add is that I have found in swimming whitewater (deep, bigger water) a swimmer can turn on their stomach and face upstream and have a good position for breathing. So you can essentially "ferry" swim and be in a pretty optimal breathing position.
Also, in the biggest of whitewater, boat rescues are difficult to impossible and swimming your tail off is a better option than crawling on the back of a team member's boat. Most of the time.
I agree that Class 5 swimming for practice is generally not "smart swimming practice." Too dangerous.
I guess the point of my previous post is that in really big and continuous whitewater, it seems that most of the time, the best option is to use the energy you have to swim to shore as soon as you can.