That's great that your yorkie swims!! I LOVE it when yorkies do the same things that all the big dogs do. They're not just little lap dogs...they fetch, they growl, they jump (mine goes real high), they climb, they chase....
I've taken my Zoey to the "beaches" here on Michigan Lake, and she doesn't seem to know what to do. She seems afraid of the waves and barks at it when they approach her!
I did try to fill up the bathtub once, and I swear she swam for a second, but pretty much jumped right out. She's definitely not a swimmer!