Hey there!
It's because of light refraction in water. When light passes through most things, it goes at it's normal speed - the fastest in the universe. However, when it passes through water, things are a little different.
When the light enters water, it drastically slows down in speed. When this happens, the direction of the light slightly alters and makes it look like objects are in places they're not. That's waht can make fishing so hard in clearer waters. When you see a fish, you think it's in the place you see it in but it's really not.
Hope this helps!