At what minimum distance must it strike the air-water interface, relative to the spot on the surface directly above it, in order that the light does not exit the water?

Where Angle of Incidence is greater than critical angle i.e., 48.5 degrees for water-air refraction...

Explanation:

For a light beam to does not leave the second medium i.e., water, the angle of incidence must be greater than that of a critical angle. The total internal reflection then occurs and the light beam doesn't exit the medium as a result...