Darter (Anhinga melanogaster) is also known as Snakebird. The bird floats low in the water such that only the head and neck are above the water level, looking like a snake moving in water.
The bird hunts small fishes underwater, sometimes diving into the water to do so. As birds are generally light and float in water, the darter, as well as cormorants, need to increase their weight to be able to easily move underwater. This is achieved not having their feathers waterproofed, so that they become heavy with water.
However, once they emerge from the water, they need to dry themselves. Thus these birds sun themselves with their wings stretched out. They also squeeze their feathers through their bill to remove excess water and repel water with oil from their enlarged preen gland at the base of the tail.
Images by Johnny Wee, taken in Perth, Australia. Top image shows a Darter hanging out its wings to dry, image below shows it with its bill manipulating its preen gland.