[shkkmo]

When diving (as opposed to using a submarine), your body reaches the same pressure as the water surrounding it so there is no differntial to feel.

The effects of that pressure can change the way that our bodies interact with the gases disolved in our blood.

Nitrogen/Gas narcosis occures (with a normal air mixture) due to the higher partial pressure of those gases in your blood and cause an effect similar to inhaling nitrous oxide.

At even higher pressures, oxygen (in standard air mixtures) reach a partial pressure in your blood that becomes toxic, can cause severe damage and trigger seizures. (Which at depth will likely be fatal.)

Drivers who go deep must switch between different air mixtures as they descend and ascend through different pressure. This "tech" diving can be very dangerous and requires detailed training and planning to do safely.

I am no biologist but would imagine that animals who dive deep have developed specialized metabolisms to deal with the changes in partial pressure of gases as they dive deeper. (Edit: it looks like diving air breathers tend to have mechanisms to reduce the gas exchange between their lungs and blood so that the partial pressure of gases in their blood doesn't go up as they go down)

Decompression is a different issue and has to do with how these high partial pressures of gases (especially nitrogen) work their way out of tissues without coming out of solution and forming air bubbles.