A dry bag is not a bad idea, but I think you are over estimating it's ability as a safety item. You wouldn't really float unless it was filled with light items, and filled with air. If you had your drill, battery, and more in there, it would not float. I just don't see a person dumping out the contents, filling it with air, and then using it to float. You could have used that energy to just get out of the water. You would be better off clipping a foam throwable flotation seat onto the pack, and using that.
The dry bag, fine idea. For less demanding tasks, those 2.5 gallon Hefty bags are amazing and cheap.