Why can't we see underwater if most of our body is made of water?
False assumptions and false reasoning lead to false conclusions. Your eyes do what they were meant to do. Nothing more...nothing less. We also don't see infrared and ultraviolet. Some don't even see a difference between shades of red.
I don't know about you, sweetie, but I have no trouble seeing underwater. Most human beings do not. And many aquatic creatures actually get around well by a kind of radar or sonar, NOT acute eyesight. That's what they possess that we don't, not better eyesight.
Imagine being able to see clearly underwater but then being nearly blind on land. Gotta choose one or the other. Aquaman.
Just go underwater and open your eyes. It will all make sense.