It might have been filled with mineral oil, those external enclosures often setup that way so that the enclosure is less extreme to manufacture. Not sure if that would work for camera lenses though unless those were also filled.
As someone who was on the product team for a 6000msw video camera, it's probably not filled with mineral oil. I doubt anyone makes subsea camera bodies/optics completely from scratch, and off-the-shelf units are not designed for pressure. In ours we used Sony camera internals, the enclosure was atmospheric, filled with dry nitrogen to reduce condensation, and the sapphire lens was designed to resist 12500psi and reduce distortion from the air-->sapphire-->seawater interface.
Hmm ok that would have explained why the SD card wasn't damaged if it were because when the vessel is filled with mineral oil it wouldn't have imploded like the main body of the craft.
