I sometimes do leave my pack and go on a dayhike or climb a point. When I do, I take the 10 essentials with me.

I think it's a little ridiculous to imagine anyone letting their pack fall into a river, but I do know many folks undo the hip belt while crossing them with the notion that if they do go off balance, that heavy object won't drag them under - I don't bother undoing the belt because I feel more secure with the pack properly balanced on my body, personally. And no, I've never known anyone who lost their pack completely. I have yet to hike with someone who is that ridiculously careless.

I actually keep most of the ten essentials in my cargo pockets on the pants I wear, as well as my keys and credit card/cash. It's not that hard to have them at hand. That way if I want to stash the pack and head up the rest of the way to a peak, it's not a huge deal digging stuff out of the pack - take the water and go.