Very good. Hooded Mergansers are diving ducks, chasing fish underwater. For that, they need excellent underwater vision. If you have ever opened your eyes underwater without goggles, you noticed that everything looks blurry.

Light bends as it travels from one substance to another substance with a different density. The bigger the difference in density, the more it bends. Because water is denser than air, eyes that focus clearly in air will see thing blurry underwater.

Mergansers and some other diving ducks have special muscles in their eyes that can change the shape of the lens. This lets them see clearly in air, and then reshape the lens to let them also see clearly underwater.