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 that have "down" are going to get soaked thru. Down does not hold the oil or layer against water getting past. Duck down is more like hair and will soak down and weight the birb down to the point that it cannot swam and stay afloat.
Ducks loose some of the water-proofing during a "molt" where old feathers get replaced by new. This is when they get their "upgrades" in plumage and coloring. They always have some down underneath the feathers and that can hold water when they are molting. Ducklings are not strong enough swimmers to manage when they become soaked, so some do drown when they are young.
If your adult duck thats fully feathered gets soaked easily and does not shed the water quickly then he/she might have a gland problem. The "preen-gland" is located at the base of the spine right in front of the tail. You will see the duck pick at that area to wet his beak with oil then spread it around. If that gland is plugged up you may have to clean it by spraying warm water from a shower massage or applying a warm wet rag to the area then wash. The oil can collect dander and dirt and plug up the skin where it cannot pass the oil thru to the skins surface.
A warm bath....is basically all it takes to fix a plugged gland. You can feel the bump under the skin right before the tail.
If you ever "pet" your ducks or hold them, you'll notice that your hands feel chalky or dry after petting or stroking their feathers...thats the oil getting on your skin. Its so thin that you do not feel any wettness. Its dry oil like "Neats-Foot" oil for water proofing leather boots and shoes. I have no idea what a "neat" is or how they get oil from their feet.