Not only the pressure differential, Cl- is amazingly corrosive and in seawater it’s accompanied by its pals I-, F-, animals and plants that will use the tiniest foothold to grow…
A naval engineer has to know much more about materials and their protection than automotive and aircraft ones, they deal with a much more agressive medium.
I guess the word* easier *was not useful. I think the undersea habitat would cost less, counting the costs of placing the habitat on the moon initially, along with the costs of all delivering all supplies . That would probably exceed the cost of the undersea habitat. If we’re talking about maintaining 50 people for 25 years on the moon, that’s a lot of supplies, or an enormous self sustaining habitat.
To literally build the habitat on the sea floor I guess means building an island with a shaft going down to the sea floor. It would be easier to use an existing island and dig a shaft to that depth. Air lock systems in the shaft could maintain sea level air pressure. I don’t know how harmful rapid air compression from an airlock failure would be. The habitat could be located in the middle of a continent, and have virtually no chance of being breached by seawater. If ancient dry seabeds could qualify, there are several places in the world where you only have to pitch a tent. All this presumes the idea of building a seafloor habitat has nothing to do with interacting with the real undersea floor in any way. When you do that, you get to close to the possibility of water infiltration. However in the island scenario, with a shaft that allows high pressure air to escape, and sufficient safe guards to minimize the intake of water, maybe catastrophic failure becomes less likely.
Well, now you’re talking about living at the bottom of a mine, which I think is clearly not what the OP is interested in, as in the specific words
Though I suppose Nebraska is indeed well away from the continental shelf. 
ETA: That second paragraph about building an island is a way to create a potentially safe sea floor habitat that is slighty less useful than building a smaller totally submerged but less safe sea floor habitat. Neither has any significant use at all because we have robots. But building an island for an undersea habitat could prove more expensive than putting a habitat on the moon and maintaining it (also useless as far as I can tell).
Yes, I was taking the OP’s words literally. Although if you read my followup, I don’t see the point of what the OP is interested in, whatever that would be.
You’re certainly free to leave if you have nothing to contribute. Apparently you believe you’re being clever in some way, but boredom gets the best of me before I can sort out just how you think so.
The smart scenario is to go the intermediate step, from Near Earth Orbit like the ISS, to a fully sustaining colony on the moon. So instead of sending up rocket after rocket with buildings parts and food, you send up one bulldozer (disassembled) and a bunch of seeds and earth, and the colonists build their own buildings and grow their own food. The Earth I and II geodomes were steps in that direction.
The only problem I would see is in case of something going wrong: you can’t simply send a rocket to the moon right now, you need a window because of gravity. But park the ISS in moon orbit instead of Earth, and wait till we’ve perfected nanotubes for a space elevator, and response time could be quick, too.
Likewise, rescuing somebody from the ocean floor is easier today because we’ve portable pressure chambers and so can deal with depressurization at ease.
This is a good reminder: where does the power come from? For the deep sea habitat, you probably use either nuclear, or maybe wheels (no, I’m not kidding - the gulf stream has a speed of 2.5 meter per second. You would put the habitat some safe distance away, of course, and lay a long enough cable.).
Maybe your topside rescue station (permanently installed) could use solar and send down electricity per cable, but ideally a colony/ habitat should be self-supplied. That leaves nuclear, with all the inherent problems.
The moon habitat, on the other hand, has half of the moon for space to put solar modules to generate electricity, and half the dark side to radiate away heat.
Am I the only one thinking that we regularly send habitats to roughly “greater than 200 feet” with amazing regularity. mrAru was a submariner for 20 years. he guesses he has roughly 6 years total at depth.
Would it actually be necessary to keep the internal pressure of a deep ocean habitat at 1 atmosphere? With the proper gas mixture what is the maximum pressure a human can tolerate for extended periods of time?
If you can keep the pressure equalized you eliminate some of the problems with the ocean floor base. (Yes, this thread got me thinking of The Abyss.:))
We also send people into space regularly, but the ISS is still not a self-contained habitat or colony.
I don’t know about the second question, but 10m water = roughly 1 atm. 10 meters down, you’re at 2atm (the atmosphere itself plus the water); 50 meters down, 6 atm; 500 meters down, 51 atmospheres. That’s a lot of pressure…
Regarding the difficulty of building a structure able to resist the high pressure of oceanic depths :
Since, as everybody knows, we’ve…err… sent once, quite a long time ago, a sub in a very deep oceanic trench, I must assume we’re able to build such a pressure withstanding structure. I guess a sub isn’t the same as a colony, and some minutes not the same as years, but nevertheless, it’s some indication that this presure isn’t (and hasn’t been for decades) an impeding issue, no?
(every tidbit of information someday comes in handy)
Yeah but IIRC, that particular “sub” didn’t have people inside, the catch is the people.
The pressure is too great. Supposedly the cause of death when a submarine suffers a massive hull failure is incineration. The incoming water is under such pressure that it compresses the air so much and so quickly that the temperature rises to thousands of degrees. Of course a millisecond later the water reaches that point and puts out the resulting fire. The key problem is the change in pressure. Explosive compression as it were.
If clairobscur is thinking about the Trieste, then it had room for two people.
The first one did: “was first explored in 1960 by Swiss scientist Jacques Piccard & US Navy Lt. Donald Walsh in bathyscaph “Trieste”, a US Navy owned submersible manned vessel (Designed by Jacques Piccard’s father Auguste) which set a record by diving to a depth of 10, 900 meters (35,810 feet).” Later trips were with ROVs.
Ask mrAru how close he ever got to 12,000 feet down. All that depth stuff is still classified, AFAIK, but I’m doubtful he ever got close to 2,000 feet down.
Its somewhere on the order of 1000 feet (maybe 2000) but thats pretty short term. I suspect long term (weeks, months, years) is less than that. Thats way short of the 10 to 20 thousand feet that would probably satisfy the OP’s "goal. Of course its always possible that some drug/chemical could be discovered that allow long term exposure to such a thing.
Perhaps you should follow your own advice.
ETA: I see you are the OP. I offered information based on your hypothetical. If you’re not interested in answers to your question, why did you pose it?