the 'down' feathers or hair is like a sponge...Down soaks up water very well...as the duck becomes totally water logged it gets harder to stay afloat and it will eventually go under if the birb cannot find a place to stand or rest its legs...
I've seen one of mine struggling to paddle and stay...
Ducks are not water-proof at all.
They spread a dry-oil around their feathers so that water is repelled and cannot penetrate their multi-layered feathers. Alot like the shingles on the roof keep water out, except the duck has an oil film to help.
Ducklings that are not fully feathered in...