Wolf Gnards had an excellent article a few years ago on this same subject, and through the assumption that each coin is the equivalent to a dollar and is roughly the size of a silver dollar, McDuck's vault would be worth upwards of $27 trillion. Using Wolf Gnard's calcuations, however, and assuming that each coin is the same weight of a silver dollar (26.73grams/.94oz) but made of solid gold, then McDuck's stash of petty cash would be worth $44 Quadrillian (which are one thousand million millions).
However, neither article answer the matter at hand. I don't really care about how much Mister Scrooge is worth, I just want to know how much money you need to be able to swim through it S-McDizzle style-- you know, if gold coins together formed more of a liquid and not a hard, floor-like surface.
A fairly conservative private pool of 12 x 24 feet with an average depth of 6 feet, would hold 14 million coins (assuming again that they have the same dimension as a silver dollar), and made out of solid gold would equate to $24 trillion. Ugh, dream on. I don't even want to know how much you would need for an Olympic size pool...
Okay okay, how about something very modest? What would it take to fill a large Kiddie pool, perhaps something around 26 cubic feet/200 gal? A paltry $373 million! Totally affordable! And if you're a billionaire, you can fill your entire hot tub and still have your lady money on the side. I'm hoping that someone has done this! If not... Well, ballers of the world, I am disappoint.