Birds are very well adapted to aquatic environments, as you can see in this short list of facts about the swimming ability of birds.
Most aquatic bird: the Penguins, who spend 75% of their life-span out in the open sea.
Fastest underwater swimming bird: the Gentoo Penguin, who flies through the water at 22.3 mph (36 km/h).
Deepest dive for non-flying bird: the Emperor Penguin, who has reached depths of 1,772 feet (540 m).
Deepest dive for a flying bird: the Thick-billed Murre, who has descended to 689 feet (210 m) below the ocean surface.
Deepest dive for a flying bird under 7.4 ounces (210 g) in weight: the Peruvian Diving Petrel, which has been recorded at a depth of 272 feet (83 m).
Longest time submerged: the Emperor Penguin, which has remained submerged for up to 18 minutes.
Ancient Murrelet chicks can swim underwater using their wings and feet when they are only 3-4 days old.
Source of these facts: The Bird Almanac, by Dr. David M. Bird