A point source is located at a depth h beneath the surface of water...

Light rays from the point source at depth $h$ will escape the water surface if their angle of incidence at the water-air interface is less than or equal to the critical angle $\theta_c$.

The critical angle is given by $\sin \theta_c = \dfrac{1}{\mu}$.

By symmetry, the locus of all points on the surface where the angle of incidence is exactly $\theta_c$ is a circle.

The radius of this circle is $r = h \tan \theta_c = \dfrac{h}{\sqrt{\mu^2 - 1}}$.

Thus, the light escapes through a circular area on the water surface.

The centre of this circular area lies directly above the point source.