In general, when a beam of light (the incident beam) hits the interface between two transparent materials, such as air and water, part of the beam is reflected and part of it continues through the interface and on into the other material. The light beam is bent, or refracted, as it passes from one material into the next (click to enlarge the diagrams below).
The farther the beam is from perpendicular when it hits the surface, the more strongly it is bent. If the light is moving from a material with a low speed of light into a material with a higher speed of light (for example, from water into air), the bending is toward the surface. At some angle, the bending will be so strong that the refracted beam will be directed right along the surface; that is, none of it will get out into the air.
Beyond that angle (the critical angle), all the light is reflected back into the water, so the reflected beam is as bright as the incident beam. This phenomenon is called total internal reflection, because very nearly 100 percent of the beam is reflected, which is better than the very best mirror surfaces.
The critical angle for water is measured between the beam and a line perpendicular to the surface, and is 49 degrees.