If humans... cant see air. . . Does that mean Fish cant see water?

IF you as a human is under water you don't see the water.

Interesting question! This is going to keep me busy for a while.

Fish can't see the water any more than we can see air. But what we can both see is the transition from one to the other which is why a pencil will always look bent when it's half underwater. Try it for yourself, fill a pan with water stand a pencil up in it and look down the length of the pencil and you'll see the bend. Fish have a much wider field of view than humans so their entire vision would be distorted around the edges but they'd still see that bend. :-) x

what about the sun rays piercing the water? that must be seen.

Human vision is about light-reflections.
You can just read about how it works.

Air can't be seen. Water is.

Or eye-vision, more exact.

Depends on how their eyes work