The whole swimming in coins thing was a plot point in The Life and Times of Scrooge McDuck and a few other stories by both Barks and Rosa, I think. The short explanation is that he's just had that much practice. I don't remember anyone else ever being able to do it (Flintheart Glomgold might, but I'm not sure).
Rosa's explanation (also in The Life and Times of Scrooge McDuck) for the money in the bin was that those are the cash that Scrooge has personally gathered by travelling around the world, and all the new money goes elsewhere, or is liquid. But that's just Rosa. I think Scrooge knows how much there is, anyway, because he's obsessive like that.