I'm pretty sure the coin swimming is just a special ability Scrooge has. I recall an episode of Duck Tales where he took the boys to the Money Bin and dove in for a swim, then Huey Duey & Louie jumped down, landed hard on the coins and were all "how does he do that?" while they rubbed their heads.
Oh, and I'm not sure who's richer between Bruce & Tony, but Tony sure seems to be enjoying life more so I'd assume him. Then again, maybe it is Bruce. You know the saying- mo' money, mo' problems.