Penguins may have as many as 11 feathers per square centimeter. They are short (about 40mm) and dense and offer good wind and water resistance. The feathers are coated with a water repellent oil secreted from a gland near the tail, similar to ducks.
To aid swimming the feathers are flattened against the skin, partly by muscular control and partly by water pressure. This reduces their buoyancy and improves hydrodynamics. They instantly expand on coming out of the water so that insulation is maintained in the cold air. The compression of the feathers may improve their water resistance.
Amazing fact: An emperor penguin was recorded diving to 540m (1772ft)!