system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't we see well underwater? What is different about the eyes of aquatic animals?
gpt
The key difference is in the lens of the eye that handles focusing the image onto the retina.  When focusing, the "work" is done as light transitions from one material to another, as the index of refraction changes, which causes the light to bend.

Outside the water, this is (roughly) air -> lens -> vitreous humor (the fluid in most of the eye).  Underwater, the air is water instead, and the difference between the index of refraction of air and the material of the lens and water and the material of the lens is much greater.  So outside the water you need a much less round lens to do the focusing.

Human eyes are adapted to focus well in the air, while the eyes of aquatic creatures do better under water.  This is also why if you put goggles on, you can focus well underwater.