Say we have a jar of water, Is the angle of incidence, the angle that the the light enters the water, the angle above the water or the angle below the water?

Angle of incidence is a measure of deviation of something from "straight on" and may refer to: Angle of incidence (aerodynamics), the angle between the chord of the wing and the longitudinal axis of the fuselage. Angle of incidence (optics), describing the approach of a ray to surface.