It's the speed of light in a vacuum that is fixed according to relativity, not the speed of light in matter. Light goes about 2x10^8 m/s in water, but that has nothing to do with relativity. The following FAQ may also be relevant.
FAQ: Is the c in relativity the speed of light?
Not really. The modern way of looking at this is that c is the maximum speed of cause and effect. Einstein originally worked out special relativity from a set of postulates that assumed a constant speed of light, but from a modern point of view that isn't the most logical foundation, because light is just one particular classical field -- it just happened to be the only classical field theory that was known at the time. For derivations of the Lorentz transformation that don't take a constant c as a postulate, see, e.g., Morin or Rindler.
One way of seeing that it's not fundamentally right to think of relativity's c as the speed of light is that we don't even know for sure that light travels at c. We used to think that neutrinos traveled at c, but then we found out that they had nonvanishing rest masses, so they must travel at less than c. The same could happen with the photon; see Lakes (1998).
Morin, Introduction to Classical Mechanics, Cambridge, 1st ed., 2008
Rindler, Essential Relativity: Special, General, and Cosmological, 1979, p. 51
R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters 80 (1998) 1826, http://silver.neep.wisc.edu/~lakes/mu.html