
  There was an interesting article in Scientific American some time ago
about breathing liquid. (It was a few months before _The Abyss_ came out.)
As far as I can remember, they mentioned three things that were difficult
to do at once with a substitute breathing fluid:
  - low viscosity --- if it's too difficult to force the fluid in & out 
    of the lungs, you can't extract enough oxygen to power your own
    breathing effort (let alone anything else)

  - diffusion rate --- obviously, not all the air in your lungs is
    expelled when you breathe out; and the part that isn't expelled
    is the part that's nearest the walls of the alveoli. (alveolus?)
    So the trip from the blood vessels to the new air has to be done
    by diffusion of the gas through the fluid. Apparently oxygen
    tends to diffuse more readily than CO2, so even if you can get enough
    oxygen in, you might not be able to get enough CO2 out.

  - oxygen/CO2 capacity --- you have to be able to dissolve enough
    gas per unit volume. 

  Oh, and of course, your new breathing fluid must not irritate the lungs
or interfere with their healing or anything like that... 
