Well, my experience is specific to subsea oil and gas extraction systems. For those you have divers and ROVs do the vast majority of any underwater work and the control systems are all above sea level, typically in the large to giant offshore rigs.
I am not an expert, even after the 8.5 year, as it was a large company and I was a cog in the gears as an instrumentation specialist. So I was working with subsupplier manufacturers to incorporate sensors, valves, meters, etc, etc into the systems. Additionally the expertise to design subsea habitats was not something I ever had to deal with since subsea oil extraction doesn't make underwater habitats for their systems! As far as I know, there are no such structures existing as the cost would be enormous and unnecessary since you can do everything except some rare physical repairs from above sea level control systems.
Since the cost of shutting down an oil field to affect repairs is absolutely enormous and the subsea repairs can be quite complicated, the systems are usually spec'd for 25 to 30 year lifespans with double redundancy on any possible failure/error points (for example: if you have a valve break on a Texas desert rig you just get a new one, shut down the line for 15 minutes and perform the install. On the same situation on a subsea would require many hours (and possibly millions in time revenue) and have to be done in a hostile environment by ROVs and divers. Plus you need the specific replacement parts that require the correct materials, coatings, even fasteners, etc in addition to all of the actual sensor and operational functions. Everything is overbuilt/over-engineered.
So, the facility in Underwater isn't reality and exists only as a plot device to make a point about the potential dangers of screwing with Mother Nature.
You mention Deep Blue Sea. That was a scientific research facility. While I can't claim expertise there, I can say that generally speaking it is also a fictional fabrication. The actual underwater habitats that I am aware of are essentially large skids with a bunch of individual metal capsules on them which are built above ground and lowered into place. While they will be anchored to the bottom, they are not technically permanent in the sense that detaching any foundation anchors can allow them to be moved, even lifted entirely out of the sea. So this lends to reason that it is more like being in a spaceship: a small, contained environment. No large concrete walkways, or huge open structures.
Look at it like this: Underwater environments are much like outerspace with some of the main differences being that outerspace has vacuum pressure, while undersea has exponential pressure by depth, and the corrosive elements of the water environments.
So even with the possible scientific research applications, it would be rare to ever have any permanent subsea personal structures at any significant depth ever. The pressures you are dealing with, the corrosive environment, the inability to guarantee the stability of the foundation, and the lack of necessity to keep the personal at these depths, etc makes this fiction (I'm sure there are rare exceptions). Just build your surface structure with a conduit to the depths if that is somehow necessary instead of trying to way overbuild a structure that can withstand the depths for decades.
Sorry about the rambling. I should have condensed it more. But I do enjoy sharing what little knowledge I do have.
Jeebus. Thanks for the knowledge share, man. WTF do you mean by 'large skids with a bunch of metal capsules'?!