A mostly fluid filled skull is not an indication of brain death, in fact there are cases of people with most of their brains spread in a thin layer against their skull. [1]

The name for aware but apparently comatose cases is locked-in syndrome. [2] A nightmare scenario for sure.

[1] http://www.rifters.com/crawl/?p=6116

[2] https://en.m.wikipedia.org/wiki/Locked-in_syndrome