Two mechanisms are commonly used to allow objects submerged in water to resist the pressure of the water column pressing down on the object, these are:

a-the elimination of voids or air spaces within the object, with a pressure equalization device, which eliminates differential pressures, making the depth infinite, and

b-the construction of reinforced objects which can physically resist the pressure to some breaking point. The pressure hull of the submersible is an example of this way of dealing with the problem.

If you look at a submarine, both are usually used, pressure hull (which remains dry) is built to resist pressure to a point, the ballast tanks are not built to those levels of strength, so they are either full of water (submerged condition) or filled with air to match the sea pressure, to surface the boat, and to run on the surface. To dive the tank is refilled with water.

People have depth limits due to voids full of air (ears, sinuses, GI tract, and so on). Voids under fillings in your teeth are a very painful example of the ensuing pressure change trauma.

Organisms which live at depth tend to be free of voids, or air spaces, and are not subject to the pressure related issues. On the other hand, rockfish brought up from depth generally evert their air bladders, and die.


USN "Balao Class Fleet Boat" Test Depth 400 feet/Maximum Depth survived a 1000+ foot excursion (the Chopper) in the Caribean.