Before my last Grand Canyon trip, I bought a book Death in the Grand Canyon
at the little store up by the Navajo Bridge. An interesting comment in the book was that most boating deaths were the result of cardiac arrest, not drowning.
Now, maybe you'd just have a heart attach if you realized that you were heading into The Ledge Hole, but the analysis in the book attributed the cardiac arrest to the cold water. I think a shorty wetsuit might be in order, but I'm not sure a person could stand to wear one during the summer season.
Plan B, if you can't wear a wetsuit to prevent the cold water from causing cardiac arrest, would be to carry a defibrillator. I just bought this AED (automatic external defibrillator) from an ad on craigslist.
I'll feel a lot better having this along.