Specular reflection occurs where the light ray in the glass strikes the reflector. If no light is to enter the water, we require that there be reflection only. Which phenomenon prevents the light from entering the water?

The critical angle phenomenon.

Explanation:

Critical angle in optics is the smallest angle of incidence of a wave, that will give total reflection of the wave. This phenomenon occurs at the boundary of two medium, where light will normally move from one medium to another.

To prevent light from entering the water, the angle of incidence of the light incident on the water must exceed the critical angle.