Enlarge / Gentoo penguins are the world's fastest swimming birds, thanks to the unique shape and structure of their wings. (credit: Priya Venkatesh/CC BY-SA 3.0) Gentoo penguins are the world's fastest swimming birds, clocking in at maximum underwater speeds of up to 36 km/h (about 22 mph). That's because their wings have evolved into flippers ideal for moving through water (albeit pretty much useless for flying in the air).