the liquid they "breathed" in the movie was a HOFC or Hyper Oxegenated Fluro Carbon.. I was told that the navy had tested it on a diver but the FDA only permitted its use on one side of the lung, but aparently the rat scene was for real..
The physiological probems are the density of the liquid and the bodys inability to physically pump it. Also, removal from the lungs was a problem back then, but as someone pointed out its use is entering the medical field now, so one day...one day, we might see it in diving, you never know and I dont think it will be soon.
My favorite scenes were when the Commando flips out from HPNS..