The molecules wouldn't really be compressed at all, water is a liquid and to all intents and purposes incompressible. I was thinking more along the line of the reactions being supressed to to pressure effects. This does have an effect of reactions and some will only take place within a narrow band of pressure/temp constraints.
 If you were to take your time you would end up still Peter shaped and whole on the bottom, you just may not be alive!
I don't understand what you mean by the gravitational pull, not sure what you mean if we are talking about earth
 gaseous oxygen is still around at the bottom of the ocean dissovled in the water, as far as I know it is more temp that causes it to liquify. liquid based breathing systems still wouldn't solve the narcocis effects though and you would still have deco obligations as you would have to breathe something as a dilutent and this would saturate your tissues big time