First, the introduction. (Apologies, but it's important and relevant.) *Note: The fact that the diver is breating compressed air has a profound affect on the body if decompression standards are not met when coming to the surface having spent time at depth. Divers who do not decompress properly can (and have!) died as a result. *Note: The breathing of "regular" compressed gas (air) at "great" depths will increase what is called the partial pressure of the gasses breathed to the point where nitrogen narcosis (the "rapture of the deep") can set in. It will intoxicate and disorient a diver who "goes deep" to the point the he may end up drowning because his cognitive faculties have been compromised and he "spaces out" down there. *Note: The failure of a diver to "equalize" the pressure across his eardrums while descending can result in the rupture of the eardrum(s) and can destroy a diver's hearing in one or both sides. The issues cited are serious. Very much so. They can be life or death. But the wonderful thing is that they are very manageable. The well-developed training and the superior equipment used by divers today make the sport quite safe. The diver controls "99%" of the risk. It's an activity that is doable by most individuals. Most of us risk our lives every day in motor vehicles and we do so without much thought. We have done a personal cost/benefit ratio and have decided to risk getting in the car. We have mastered the basic driving skills and we venture forth having had a little (or a lot) of practice. No problem. That said, let us (finally!) address the question. Other than the issue of decompression, the issue of using "standard" compressed air (as opposed to mixed gas) at "excessive" depths, and the issue of equalization, the body is unaffected on the whole by the compression of the water. Get a balloon and fill it with water to make a water balloon. Be sure to get all the air out. Take the water balloon and put it in a bucket of water and hold it on the bottom. Does it get smaller? No. It is almost completely incompressible. Except for the inside of the lungs (and the interconnection between the airways and the inner ear), the body is the same way. The materials out of which the body is made, and the fact that much of it is water, make it "incompressible" with the exceptions noted. When using air to Scuba dive, air taken into the lungs is taken in at "increased" pressure because of the action of the regulator. The increased air pressure will insure that the lungs cannot "collapse" due to the pressure of the water. The lungs and inner ear aside, the body is largely unaffected by water pressure. The diver breathes normally and suffers no ill effects (with the exceptions noted). Some feel for what it is like can be had by going to the bottom of a swimming pool. In the shallows, an individual submerging will feel weightless because buoyancy works against gravity in water. In a "regular" pool, the diver may have to equalize a bit to get on the bottom, but once there, the body is not "crushed" by the water. The lungs are a bit compressed, but that is all. The diver does not notice. The pressure of the water is a factor that is "transparent" in the experience. It goes by will little heed paid to it. (It is water's resistance to the diver's movement which gets all the attention. It is a surreal experience to anyone diving for the first time - as well as returning veterans - to get in the water and become one with the element and its denizens. If an opportunity to dive presents itself, particularly to someone who had not done it, it is a chance to be jumped at. There is nothing like it anywhere. Nothing.
As you go deeper in the water, the pressure of the water outside of your body squeezes you, and would crush your lungs (they are hollow, you know). However, divers are supplied with air that is at the same pressure as the water around them- pressure inside equals pressure outside- so no collapse. HOWEVER, the nitrogen that is in the air, which usually does not enter your bloodstream, CAN be pushed into your blood by the higher pressure. This can cause "the bends" if you stay too deep too long, and do not decompress as you ascend. You must also be sure to exhale as you ascend- or the high pressure in your lungs, no longer being balanced by high water pressure, could make your lungs expand and tear.
At 4 meters below the sea level, the pressure exerted by the water column above the diver would be approximately 0.4 atmospheres higher than atmospheric pressure at the surface. Therefore, the expected pressure of air in the diver's lungs would be the sum of this increase and atmospheric pressure.
Changes inside the diver, such as changes in buoyancy or density, can affect the diver's position in the surrounding fluid by causing the diver to either sink or float. For example, if the diver becomes less dense than the fluid, they will float to the surface. Similarly, if the diver becomes denser than the fluid, they will sink to the bottom.
Pascal's principle states that when pressure is applied to a confined fluid, it is transmitted equally in all directions. In the Cartesian diver toy, squeezing the bottle increases the pressure inside, causing the diver to sink as the higher pressure compresses the air in the diver. Releasing the pressure allows the air to expand, making the diver float back to the surface.
Implants do not affect the ability of a diver to descend deep.
The cartesian diver sinks because the diver wants to get to a place of low pressure
The pressure exerted on a diver 10 m underwater is approximately 2 atmospheres or 1.8 times the atmospheric pressure at the surface. This means the pressure is effectively doubled at this depth.
Pascal's principle states that a change in pressure at any point in an enclosed fluid is transmitted undiminished to all points in the fluid. This principle helps explain the behavior of the Cartesian diver, as the change in pressure when the diver is squeezed causes the enclosed air to compress and the diver to sink, and when pressure is released, the compressed air expands, causing the diver to rise.
Pascal's principle states that a change in pressure at any point in an enclosed fluid is transmitted equally to all points in the fluid. In the case of a Cartesian diver, when you apply pressure to the container, the pressure is transmitted to the fluid inside, causing the diver to compress and sink. When you release the pressure, the diver expands and rises due to the equal distribution of pressure in the fluid, demonstrating Pascal's principle in action.
Assuming that by 'amateur diver' you mean a recreational diver without any professional qualificaitions, the recommended limit as determined by PADI (Professional Association of Diving Instructors) is 18m (60ft) for an Open Water diver, 30m (100ft) for Advanced Open Water and 40m (120ft) for a Divemaster. BSAC (British Sub Aqua club) sets the limits as 20m (67ft) for an Ocean Diver, 30m (100ft) for Sports Diver and 50m (165ft) for Master Diver. So in answer to your question, no, you couldn't 'swim where the water pressure is more than 65 ps' unless you were a technical diver or a BSAC qualified diver, as at 165ft the pressure is around 69ps.
Increased pressure causes gas to dissolve into the diver's body fluids, such as blood and tissues, at a higher rate. As the diver descends deeper underwater, the pressure increases, leading to more gas being absorbed into the body. This can result in decompression sickness if the diver ascends too quickly without allowing the excess gas to slowly off-gas from their system.
A diver's ears can be hurt during a deep dive due to changes in pressure. As a diver descends, the pressure increases, causing the eardrum to push inward. Failure to equalize the pressure by clearing the ears can lead to barotrauma, causing pain, discomfort, and potentially injury.
Divers breathing compressed gases at depth are themselves under pressure. For each 33 feet/10 meters that a diver descends, they add approximately 1 atmosphere of pressure to their body. When they breathe gases whilst their bodies are under this pressure, the gases dissolve into their bodies tissues. When they ascend, the pressure is relieved, and gases are released. Provided that diver ascends sufficiently slowly, the gases are released slowly and no problems result. But if the diver ascends too slowly without allowing the gases to expire at a slow enough rate, then the diver will suffer decompression sickness when gas bubbles form in the diver's tissues and blood.