The significance of $c$ is not the speed of light, it is the maximum speed a cause effect relationship can propagate. This comes to mean a speed that is observed to be the same by all inertial observers, as I explain in my answer here. It is experimentally found that the speed of light is equal to $c$, because the speed of light has this transformation behavior between inertial frames (always the same) and, from basic symmetry considerations, there can only be one such speed. The experimental result that light travels at $c$ means, amongst other things, that light has no rest mass.
Now let us look at light in a medium. This is not the same thing as light; it is a quantum superposition of photons and excited matter states. There is a quasiparticle for this superposition and it is called a polariton (or several other names such as exciton, plasmon and so forth, depending on the exact nature of the superposition), it travels at some speed less than $c$, namely $c/n$ where $n$ is the medium's refractive index and you can indeed boost to a frame wherein such a disturbace is at rest relative to you.
This particle does have rest mass. Indeed see my answer here where I calculate that the rest mass of light - excited matter state superposition quantum is about 3.6 millionths of the mass of an electron at optical wavelengths for windowpane glass ($n=1.5$). That is, a few electron volts, or of a very similar magnitude to the total energy of the incoming optical photon from a frame at rest relative to the medium (but it's not the same, since total energy is not a Lorentz invariant).