In order for that to happen there needs to be a pressure gradient. Both an area of high pressure and an area of low pressure need to exist for something to be crushed.

In the case of a zombie who for the sake of arguement we assume does not breath, it would be of no consequence for it to allow it's lungs/respiratory tract to fill with water. This would eliminate most buoyancy and remove to pressure gradient that would have existed in the thorax.

Gas is compressible and affords 'crushing' at depths whereas liquid is not. with the exception of maybe some gas in the GI tract there should be a limited amount of compressible matter to cause a problem.

Our Bones are porous but filled with marrow a liquid, our brains are a mass of mostly water, both of which make them incompressable and therefor will not crush.

The argument of the Mythbusters experiment is invalid in this case due to the experiment is not trying to test the same thing. They tested decompression at depth. The body was essentially extruded into the helmet because of the pressure exerted on the body by the water around it. The pressure differential that I spoke of exist due to feed line acting as a chimney to the surface allowing the compressed air to escape and incomprehensible liquid to fill it's space.