Death-defying, NOT!! You can safely swim every rapid in Grand Canyon. I myself have swum out of the Ledge Hole at Lava Falls at a cfs of 29K, (that's a pretty high volume these days) and it wasn't all that scary. The main danger in flipping a boat in Grand Canyon is getting hit by the boat itself. You can swim any rapid, assuming your life vest is on correctly. Yes, the water is cold, but you can wear a dry suit or wetsuit. If I'm really worried about flipping I use a cam strap on the life vest, and run it front to back, so that the vest can't come off over my head.
If the movie made you think that those guys would have died had they flipped, that was over-dramatization, and it is why I generally don't like movies. It always turns out that if you already know a lot about the topic, you end up sitting there shaking your head and saying, what a bunch of pumpkin! Reminds me of that movie, Into the Wild
, where Sean Penn decided that the flatwater Lower Colorado River was just not enough, he had to have Chris McCandless running rapids in Grand Canyon. Well, if you read the book you'd know Chris never ran any rapids, because there are none to run below Grand Canyon, and he didn't go to Grand Canyon. The movie sequences showing rapids were filmed mostly at Diamond Creek, in Grand Canyon.