At sea level, 14.7 pounds per square inch (psi) of water pressure push on our bodies. Our bodies have to push out with the same amount of force to make up for the pressure from water. Humans are only able to go 3 atms into the water before our bodies are unable to compensate the for the pressure. Past this point, a submarine is needed so that humans are not crushed by the water pressure.
There are many species living deep in the ocean. These species require special adaptations that allow them to undergo large pressure changes in short periods of time so that they can swim through different depths of the ocean. One example is the sperm whale, which swims through the ocean and experiences pressure changes from 223 atms.