Humans and mammals have one upper and one lower eyelid, birds on the other hand have a third eyelid, which is called the Nicitating Membrane.
Instead of opening and closing up and down as do our eyelids, the nicitating membrane opens and closes sideways across the eye, it is used to keep their eye's surface moist and clean.
To help see underwater, many diving birds have a special window type area in the center of this membrane, it is like the goggles Michael Phelps would wear while racing for another gold medal, allowing them decent underwater vision.
Another interesting note on these bird eyes, is the size of their pupils, they are tiny, as you look through the following pictures you will see that their eyes are mostly a large iris with a very small pupil in the center.
I would imagine their pupil grows much larger as these birds dive deeper into darker depths of the water column, just as our pupils would do in dim light.
Click on the photos to enlarge, these birds have some awesome eyes.
White-winged Scoter have a beautiful pale blue iris.
Long-tail iris's can be from brown to pink
One of the most incredible eyes in the animal kingdom.