Birds that can swim include penguins, cormorants, anhingas, and other darters and alcids, such as puffins, dovekies and razorbills. These birds enter water in search of prey, but penguins can spend most of their lives in the water.
The penguin is a bird that's truly adapted for swimming. Penguin bodies are streamlined, and their wings have evolved to be more like flippers. Their coloration of black backs and white fronts makes it difficult for them to be seen by potential predators above and by potential prey below. Penguins can also see better in water than in the air.
Cormorants swim low in the water, propelling themselves with their webbed feet as they look for their customary prey of fish. Once a cormorant finds a fish, it swallows it whole. The anhinga, which resembles the cormorant, can often be seen swimming with only its long, curved neck above the water, which gives it its other name of "snakefish."
Like penguins, common puffins have the same black-and-white coloration and also swim underwater using their wings. They eat their prey of fish and crustaceans underwater, but they can carry large amounts of food back to their chicks. Unlike penguins, puffins are able to fly.