Duck baiting?
I don't think that dogs swimming in a lake can catch wild ducks by out swimming them.
However I do know why dogs roll in horrible smelling stuff. I had a dog that would roll in stuff, and I would let her swim in the water to wash it off.
She figured out, to roll in duck droppings before getting in the water. At first she would smear her whole body with it, but then she got wiser, and would just smear her head with it.
I had to stop letting her swim in the lake, when she got clever enough to get out of the water, look for bird poop, smear her head and ears in it, then paddle slowly with her head stretched out like a duck, but flat necked on the water.
When she learned to sneak up to the ducks, and got a bite of feathers, it was time to stop her from swimming in the duck's water.
Who says dogs aren't smart, has owned stupid dogs.
http://en.wikipedia.org/wiki/Dog-baiting
They did something so the duck couldn't get away.