When Voyager is taken into fluidic space against its will in Scorpion Part II, why didn't the intense density of matter instantly crush the ship?
There could be numerous reasons Voyager wasn't immediately crushed when it was translated into the Fluidic Space dimension. The primary problem is we are assuming Fluidic Space behaves like water does on our world in our Universe. One of the only things we can be certain of was Warp Drive would not function there and ship speeds were limited due to the density of the Fluid.
There are many assumptions being made about Fluidic Space in your question. Star Trek: Voyager made no effort to inform us as to the true nature of Fluidic Space, neither its composition, nor chemical makeup. We don't actually know what comprises the fluid which fills the dimension of Species 8472. We have no idea of its density, chemical properties or its molecular composition.
Fluidic Space does not appear to be exhibiting depth pressure as water does on Earth. The deeper you go into the oceans on Earth, the greater the pressure per square inch. For example at 10,000 feet below sea level, the pressure per square inch is 4,458 pounds.
We are also not privy to the physics of the dimension, so we don't know if all life was formed, created, or lives in Fluidic Space, exclusively or whether the creatures there have analogs such as planets, or for that matter whether there is even an analog for gravity in their universe.
We also don't know if Fluidic Space is under pressure, like an ocean might be, so it may be filled with fluid but no denser than water at sea level. There is also no reason to consider it's density couldn't also be less than water.
Since Species 8472's ships did not immediately explode when coming into our vacuum of space, they are either incredibly tough and did not experience any pressure differential or the difference between our two universes could be less than expected.
Voyager's inertial dampeners/structural integrity fields should allow it to, while draining some degree of the ship's power supply maintain the structure of the ship if the overall density of the fluidic space is constant and not applying significant crushing against the hull.
It is safe to assume there are similar physical forces such as gravity, since their technology, while alien, seemed quite functional and effective in our Universe. This implies Species 8472 must be familiar with the properties and cosmological constants of our Universe if they are different than their own.
Curious bonus question: If the nature of Fluidic Space were of a significantly higher pressure, why didn't any of the fluid, immediately vent into our universe? Yes, it would have vaporized immediately, but we would have seen an indication due to the volume of fluid.
I agree with Thaddeus that the name is being taken too literally and we have no idea how their physics work. It's probably not a giant fish tank. I have some ideas of my own.
Fluidic Space is like Aether
In our Universe, there is naturally no medium required for the propagation of light and gravity (unless you consider space-time itself). Before we understood that, everyone assumed there needed to be a medium for things to move through, sound through matter being the canonical model. So folks proposed "the Aether" which filled space and allowed light and gravity to move through it, yet didn't interact with anything else.
Turns out it doesn't work that way, but what if Species 8472's universe did? Aether has been attempted to be described as a fluid. Maybe Fluidic Space is filled with a medium that behaves like a fluid, doesn't interact with matter, but does interact with light and gravity. This would explain why there's no pressure (also explained by other reasons) and no drag on the ship.
It would also explain the glow that seems to come from space itself, light needs something to bounce off of to reflect back to our eyes for us to perceive it. That's why you can't see a laser unless there's dust in the air. Our space is a vacuum, there's nothing to reflect the light, so space appears dark. Fluidic Aether might reflect some small portion of light giving Fluidic Space that sickly green tinge.
Voyager detects "no stars, no planets". A slightly reflective Aether would severely compromise your ability to see any great distance, just like turning on your headlights in a fog can be worse than no headlights at all.
Their sensors don't work right
When they first arrive in Fluidic Space Torres states it's filled with "some kind of organic fluid, this isn't space, it's matter". I have some responses to deal that.
First is to make clear that "organic" means "made of carbon, oxygen and hydrogen" not "it's alive". The writers probably didn't realize that, but I'm trusting that Torres knows her chemistry terms.
Voyager has just arrived in a new Universe and make some quick scans. They're not even here on exploration, they just want to get out. Can they trust their sensors in a different universe? Shouldn't they have to recalibrate everything? Can anyone fully grasp the nature of a new universe from a quick scan?
Ok, those are cop outs because we're obviously supposed to accept what Torres says as fact. And it's obvious the universe is similar enough to ours that Voyagers complicated and highly tuned systems still operate.
"Fluidic Space" is just the region Species 8472 came from
Just because Voyager came out in a spot filled with matter and couldn't detect anything else doesn't mean the whole of Fluidic Space is like that. Voyager could have come out in the middle of a nebula! It might be some weird, anomalous region of Species 8472 that lets them jump dimensions easier.
Drawing a conclusion about Fluidic Space based on Voyager's incursion would be like Species 8472 popping into the middle of a star and calling our Universe "Plasma Space". Or appearing deep beneath the Earth's surface and calling it "Rock Space" devoid of stars and planets and spending decades studying the complex ecosystem of lithovoric microorganisms.
Fluidic Space never had a Baryon annihilation and is chock full of matter
Our universe had most of its matter wiped out early on by a nearly equal amount of anti-matter. Why there is an imbalance allowing us to have any matter at all is a mystery known as the Baryon asymmetry. If Fluidic Space never had such an event, it might have so much matter that it fills space itself! Not dense enough to be like swimming through water, but dense enough that the whole universe is a gas cloud.
To use an analogy, our Universe is filled with energy from the Big Bang known as the Cosmic Background Radiation. Does this mean we get cooked? No, it's spread so thin that it's 3 Kelvin above absolute zero. Same thing with Fluidic Space, it's full of Cosmic Background Matter that is thinly spread over the whole of their universe.
I suspect that we may never know the "real in universe" answer since I'm not sure there is one (the out of universe answer is pretty obvious), however I've been thinking about this and here are my thoughts.
Water pressure on earth is due to gravity pulling the water down. In our universe if you had a large area of space filled with fluid, it's gravity would eventually cause the liquid to collapse in on itself into a sphere. This is sort of how planets formed in our universe - clouds of dust in space slowly collapse down into dense spheres we call planets.
Since this doesn't happen, my conclusion is that either gravity works differently in fluidic space, or the fluid has no mass. Either of those would also explain the lack of high pressure on Voyager's hull. This also explains why Voyager seemed to indicate no planets, stars, or other objects in space. Planets and stars are formed by the gravitational attraction of matter in space. If for some reason that matter never coalesced into clusters, then no planets or stars would form. (How life would evolve in this type of universe I have no idea).
In fluidic space, the fluid seems to act like the vacuum in our space and somehow has no gravitational attraction either to itself or to anything else.