The constancy of the speed of light is only a 'law' within media of the same type and 'index of refraction'. The lowest possible index of refraction is that for space and equals 1.00000 and in which light has its maximal velocity. If you were an observer inside some other medium, then so long as you stayed within that medium, your physics would obey the principles of special relativity. Since the speed of light in a vacuum is higher than that in water or glass, you have the possibility of a phenomenon exceeding the speed of light in water, but not in vacuum. Charged particles that do this produce Cerenkov Radiation as the particles move faster than the speed of light in water.