Note: this was originally posted on my blog on tumblr.com. I have tried to convert some of the formatting over, however there may be some inconsistencies. Also this particular entry was written before I knew how to cite scientific sources, so sources will either be nonexistent or incorrectly cited.
Alex, a fairly minor character in Heroes, has the ability to breath under water. And given how human anatomy works, he would definitely have pressure problems, but the exact opposite of someone who can fly. The show never shows how deep he can swim (I think we only ever see him use his ability in a pool which isn’t really that deep). But I’m going to assume he can go really deep, just to look at the most extreme case.
Deep sea animals are actually mostly made of water so they are less impacted by the pressure, thankfully humans are mostly water too so that’s not a problem. Some of these animals are actually mammals, like sperm whales. These are mostly diving animals that don’t actually live at the very bottom of the ocean, but can dive to extremely high pressures. As they dive deeper the sperm whale actually collapses its own lungs to adjust for the pressure, storing the oxygen in their muscles for the dive. Just like birds, sperm whales then use myoglobin instead of hemoglobin to bind the oxygen super efficient and allow it to move around the blood stream.
Animals that actually live on the sea floor have bladders filled with oxygen that has been adjusted to the same pressure as the bottom of the ocean. If you tried to bring these animals to the surface they would literally explode.
So it seems more likely that Alex would have an adaptation more like the whales that would allow him to collapse his lungs and dive for a short period of time. The real problem here seems to be adjusting quickly to new pressures, not so much as the pressures themselves. To put this in perspective: even when scientists bring deep sea fish up for study and put them in pressurized tanks most die a few days later probably from stress but they don’t know exactly why. Adjusting to underwater pressure changes is really hard on the body.
Another problem that Alex would face would be getting enough oxygen, and this is problem that all fish face. The top of an ocean, or the top of a swimming pool, is constantly being stirred by the wind and mix oxygen in with the water. This allows the animals in the water to breath by moving the water over their gills. The oxygen is then absorbed from the water strait into their blood stream. However not far below the surface dead zones start, areas where there is not enough dissolved oxygen in the water because the water is not often moved by ocean currents. Animals need special adaptations to survive in these areas
There are also areas of the open water where warming water has caused plankton and other micro organisms to turn into giant plooms. These plooms use up all the oxygen in the environment, and then why they die they sink down to the bottom of the body of water and release even more carbon dioxide. This warms the water more and makes the problem worse. This is what caused the giant Dead Zone in the middle of the Gulf of Mexico. Nothing can live there besides whatever makes up the ploom. So a person who could breath underwater would have to strategically avoid these areas or else they would suffocate.