What enables ducks to swim on water?

The duck’s feathers have tiny projections called barbules which hook together and trap air between the feathers. The duck has air sacs connected to the lungs which help to hold air inside its body. The air between the feathers and the air in the air sacks lend buoyancy to the duck’s body and keep it afloat. The air sacs are partly deflated when the duck dives under water for food. The act of preening feathers helps waterproof them and is presumably an additional factor.