If you have ever opened your eyes underwater, you know that things look pretty blurry. That is because our eyes are adapted for seeing in air. The lens in your eye is shaped to focus light coming from air into the eye. When the light is traveling from water to your eye, the light bends differently, so things don't focus.
Mergansers can change their eyes to adapt to the refraction of air or water. That lets them see clearly underwater to chase fish, and above water to watch for predators.