According to Falklands Conservation, penguins can swim at speeds of up to 17 mph; however, they normally average between 9 and 15 mph. Penguins can also dive further and swim faster than any other bird.
Emperor penguins have an average swimming speed of 6.7 mph but have been observed swimming as fast as 8.9 mph. King and chinstrap penguin's average speed is 5.3 mph, while the fairy penguin swims much slower at only 1.6 mph.
According to About.com, penguins jump in shallow arcs in the air while swimming, which is a practice known as "porpoising." This practice covers their plumage with tiny air bubbles and allows them to reduce friction while swimming and avoid predators. Some scientists theorize that penguins engage in this practice simply because they are happy. Penguins feed on fish, shrimp, krill and other crustaceans, and their superior underwater eyesight allows them to spot prey even in cloudy or murky water.
To get the streamlined body shape needed to reach maximum swimming speeds, penguins hunch their heads into their shoulders and tuck their feet against their tails. Their wings act as flippers to propel them through the water. Their dense bones prevent them from being overly buoyant in the water. Most species of penguins stay submerged for less than a minute when diving.