Question

What is meant by critical angle of water medium in respect to air medium?

Answer 1

One kind of critical angle is that for a beam of light within water going toward the air-water interface. For angles greater than the critical angle, the beam is wholly reflected within the water and does not reach the surface. Light pipes use this to keep losses to a minimum.

Answer 2

The critical angle is the angle of incidence that causes the angle of refraction to be equal to 90 degrees. If the angle of incidence is greater than than the critical angle then 'total internal reflection occurs'.

Now to answer your question lol, (assuming the light beam is coming from the air) it would mean the angle of incidence from the air that causes the angle of refraction in water to be equal to 90 degrees.