Well, the rough summary in either case is that the person in question would be very dead, very fast, in a very unpleasant manner. But, since you seem to be looking for a more specific answer than that, letís take it one at a time. (Let me warn you ahead of time that if you have a weak stomach, you may not be interested in some of the images that this all draws to mind.) First, if a person were to wind up unprotected at the bottom of the ocean...
At the bottom of the ocean, thereís four big things that would happen. (1) Itís cold. Because itís so cold, the person would experience severe hypothermia, and the body would eventually stop working because of the cold temperature, but this is probably not what would kill them. (2) Thereís no air. You canít breath at the bottom of the ocean. If you canít breath, your body wonít stay alive for more than about 30 minutes. (Although youíd lose consciousness after about 5.)
(3) The water pressure is very high. The pressure from the water would push in on the personís body, causing any space thatís filled with air to collapse. (The air would be compressed.) So, the lungs would collapse. At the same time, the pressure from the water would push water into the mouth, filling the lungs back up again with water instead of air. But if thereís no air-filled space to be pushed into, the body would not be crushed. (Part of the problem with the old pressure suits that deep-sea divers used to use was that if they depressurized, the soft part of the suit and the entire body would be crushed into the rigid helmet. This is one of the big reasons that divers donít use suits like this anymore.)
(4) At high pressures, the chemistry of how the body works changes. If the person did have some way of getting air into their system, their body would immediately undergo whatís called "nitrogen narcosis." This happens because at high pressures, nitrogen is much more soluble in water (or blood) than oxygen is. And especially since air is mostly nitrogen, the blood would become full of dissolved nitrogen. The nitrogen would bind to the parts of the body that need to use oxygen, and the person would literally suffocate from the inside out.
Ok... now, if the person were unprotected in space. (Fortunately, this hasnít actually happened yet, so we donít really know exactly what would happen. But hereís the best guess.) In space, the three big problems are (1) no air, (2) thereís no air pressure in space, and (3) itís extremely cold (much colder than at the bottom of the ocean).
If the person were able to hold their breath, they could probably last for at least a few minutes before they ran out of air (and this would protect them from their lungs freezing, too).
The lack of air pressure would pull outwards on their body, but they wouldnít blow up the way the people did in "Total Recal." What would probably happen instead is that the blood vessels near the skin would burst, causing their skin to turn a pinkish-red color. But itís pretty hard to say if it would be the lack of air or the cold temperature that would kill them first.
(mod by mw) The cold would be a less immediate problem, because the thermal conductance of the vacuum is low. The body would gradually radiate heat as infrared waves, slowly cooling down. The other problems would be fatal first.
(published on 10/22/2007)