On one trip in the West Fork of Oak Creek Canyon, we were confronted with about a 300 yard stretch where swimming was the only feasible option. After waterproofing critical gear (like sleeping bags) and emptying water bottles, etc, our packs (the classic external frame Keltys) were merrily bobbing along, actually serving as impromptu floatation devices, while we thrashed downstream. That, of course, was clear mountain spring water, not silt, which is truly the invention of the devil.

When sea kayaking, or skiffing through surf, I use a large dry bag with shoulder straps and waist belt, and it floats at least as well as I do.