Someone mentioned on a previous page that money does not float - like driftwood for example.
However, money attached to a body would - for a period of time, obviously. We know that Cooper jumped with the money strapped to his person. If he died in the jumped, one could conclude that the money would remain with his corpse.
Another thing to consider is that any river setting - especially one as large as the Columbia - comes with a variety of factors that when accumulated can cause weird things to happen. Animals, boats, wave action, tides, rain, snow, ice, floods, wind, etc. can all combine to act on something like a body or a bag of money. There are an almost impossible of number of permutations of variables that could place that money on that beach at that time.