First of all, I tried to look if this wasn't answered before. Found this and this but neither is exactly what I need.

Some Context

There's this race of humanoid fish people. They live in cities built on the ocean floor of the planets they inhabit.

For some plot reason, they decided to come to Earth. In order to remain hidden, they built their advanced post in a very deep part of the ocean floor.

Their "city" is but a collection of domes with breathable air inside. But they're not made of glass - they're "force fields" that repel the water. This means that if you're inside, you can literally get out and swim (since there's nothing solid between the air and the water). Which brings us to my question.

The Question

Picture an amount of pressure that would be way more than what is necessary to kill a human being (I have no idea what this number is, so I'll just guess 1000 meters down).

Given this ridiculous amount of water pressure, what would such a death look like to a bystander?

For this, just imagine you're inside the dome and someone else gets thrown in the water in front of you, exposed to 1000 meters (or more) of water pressure. I want to know what happens to this guy.

Does he get crushed? Bleeds from all orifices? Explodes? If you could also tell me how bad it gets with even greater depths, that would be super. If 1000 meters is too low, feel free to increase it up as much as you can, as long as it's still Earth parameters.

You would die quite instantly.

L.Dutch's answer is accurate if you do the shift on a reasonably short timescale. But if you do it really fast, which I'm getting the impression is your goal, the story is a bit worse.

It is true that most of the body is solid or liquid, and thus basically non-compressible. Only the air volumes will be squished. However, when you do this quickly you run into fun problems with how quickly the air can be scrunched.

1000m down means you have 100atm of pressure on you. That's 1400 pounds per square inch of your body. For a comparison, you're looking at the pressure a woman puts on stiletto heels when she strikes the floor. Your entire body will be subjected to this pressure and transfer it to the air volumes.

But it can't do this instantly. It takes time. While it's happening, that pressure gradient is still an issue. Only after its done is the body pressurized enough to equalize that force.

So during those first few moments, it will literally be as if a woman was driving stilletto heels into your windpipe in every direction. Tissues would accelerate inward at a speed limited only by the ability to accelerate the water into the space as the tissue is shoved towards the air pockets. I do believe the only limit to speed here is the speed of sound in water, which is quite high.

Now... have you heard of water hammer? Water really doesn't like to be slowed down. So once the air pockets are gone, the water pressing in on you will keep going.

Remember that water cutter from my comments? This is not going to go well. But it will be swift enough to be painless.

Pressure will act on the fluids in the body, thus gases and liquids.

Liquids

Well, liquids are practically non compressible, meaning that even applying large pressures they won't change in volume in an appreciable manner. So blood vessel, eyes, brain, etc. will not be severely affected.

Gases

Gases are contained mainly in the lungs, intestine and stomach. They will shrink to match the outer pressure (this is why when one dives after a certain depth the buoyancy is negative). The lungs are pretty spongy so the shrinking per se should not be traumatic, however it will practically nullify the exchange area, leading to suffocation.

The gas contained in the intestine and the stomach will be squeezed, too, leading to the abdomen being flattened out. Since the lungs are also shrinking I suspect the bowels would move upwards pushing on the diaphragm. That's probably not as quickly lethal as the suffocation, but I am pretty sure is really painful.

The eardrum will surely be perforated by the high pressure differential between the outside environment and the inner ear. In itself this is not lethal, but again it is painful.

Summary

Summarizing, the poor guy would die of suffocation with two perforated eardrums and flattened out bowels. Which is not a particularly spectacular way of dying, but it is rather painful.

You would find it near impossible to push someone out, let alone trow him out. To "push" 80 kg man out against 10 MPa pressure would need around 800 kJ of work and a push of 50-100 tonnes if you go head/legs first.

Maybe your dome can make pockets to eject or you have special building with "airlocks" to trow him in and fill it full with water.

How happy will our lucky winner be? There is 1000 tonnes of water over every square meter and water loves to hug things inside and around it and give all that love equally to every centimeter of the surface. With surface area of 1.7-1.9 square meters you will feel 1700-1900 tonnes of love.

Such a warm hug will compress all gases, partially heating them up and then squeeze out of you with good measure of bodily fluids, while crushing your sternum, cracking your skull and many other bones, compacting your body in to more dense human-like brick.

Show of compacting human with visuals of bubbles and cloud of bodily fluids and sounds of cracking and snapping. Near instant death, if you flood chamber fast.

Speed of water flow would be 140 meters per second. If you want you could use that as water jet cutters. They won't instantly cut bones, but enough to cut and tear apart flesh like nothing.

May as well add magical swap. If you get swapped with same volume of water. Water close to instantly squeezes you with same force as stated above. All your internal cavities with air inside will be crushed to just a fraction of their original volume. At the same time few liters of water will force their way into you. Soon compressed air will bubble out of you and water around dyed slight pink with blood from torn tissues.

As a starting point here is a video of an egg at 6,000 bar
video

1 bar is 10 meters, so we are 60,000 meters underwater.

So if you look at the egg from the outside there is nothing wrong with it. The shell withstands the pressure just fine. Once the egg is cracked open all the proteins have congealed, and the egg is "cooked" but from the outside it is just fine.

The same thing would happen to a human. While internally they would be crushed, with water rushing into every orifice, breaking barriers like eardrums and filling the person, from the outside they would look not very different.

Unconsciousness would be instant as the brain stops functioning the moment cranial pressure hits a high mark. The water would most likely flow around the eyes past the optic nerve pushing them in long enough to stabilize the pressure.

Then the probably discolored human would just sink. There would be no gasses left to keep them buoyant. Chemically they would be ruined, but physically they would not look very different.