It's high tide at a rocky beach on a sunny summer day near Juneau. Chum salmon are running, and harbor seals are cruising just off the beach. One animal looks really weird, and I raise my binoculars to get a better look. I realize it's a harbor seal swimming upside-down, belly up, head underwater.
Juneau naturalist, author and photographer Bob Armstrong has spent hours watching seals and using an underwater remote camera to take video of harbor seals underwater, documenting a wide range of behavior. He said swimming upside allows them to better scan the water beneath them for fish. Seals also swim upside-down underwater, just over the seafloor, searching for crabs and flounder. They also sleep underwater, maintaining a neutral buoyancy near the bottom, or lightly grasping a rock or log on the seafloor, and a single breath lasts 20 to 25 minutes. They bob to the surface for a quick breath and sink again to continue their nap.
Their eyesight underwater is very good. A harbor seal's eyes glow when struck by light, for the same reason cat or deer eyes shine in the dark in car headlights or a flashlight. They have a reflective layer in the back of the eyeball called the tapetum that directs more light into the retina, enhancing their vision in low light conditions.