What are some permanent underwater structures (meaning, they physically rest on the ocean floor and they were built to do so)? I know of one underwater hotel off the coast of Florida (though there may be more), and of one underwater hotel on the drawing board for off the coast of Dubai. Surely there must be some research stations & whatnot???
I asked for your title to be fixed.
This is one http://www.mrdf.org/mul.htm
here’s another:
http://www.uncw.edu/aquarius/about/about_location.htm
Good search terms were undersea lab
http://www.google.com/search?num=100&hl=en&lr=&safe=off&q=undersea+lab
Jim
By this definition, all sorts of offshore drilling platforms qualify. Do you wish to stipulate that they also have nothing sticking above the surface?
Yes, yes I do. Thank you for pointing this out.
Why, what’s wrong with it?
I think artificial reefs fit your definition - they are usually scuttled ships, but they are still man-made objects intentionally placed on the ocean floor.
Also, IIRC some undersea tunnels are actually metal structures lying on the ocean floor.
And how about submarine communications cables?
Sturctures instead of structures that’s all.
Various fish, lobster and crab traps fit the bill 
But they were not “built to do so”. so they clearly don’t qualify.
Indeed, although the material is not always metal. An example would be the BART (Bay Area Rapid Transit) Transbay Tube linking San Francisco and Oakland.
I should have been more specific: I mean, structures where human beings move around and whatnot, without the aid of SCUBA gear or artificial respiration.
That fairly effectively limits the category to tunnels.
Off the top of my head:
BART’s Transbay Tube
The Chesapeake Bay Bridge/Tunnel
The Chunnel
There are countless other underwater road or train tunnels.
If anyone’s building piers or bridges right now, there’d also be caissons.
The Chunnel was bored through rock beneath the English Channel, and thus violates the OP’s criterion that the structure should rest on the “ocean floor” (of course if the OP is strict about the body of water being an “ocean” then many of the previous answers – including the OP’s Dubai hotel – are disqualified).
I would think that the sunken-tube type of tunnel (like the BART Transbay and the Chesapeake Bay Bridge-Tunnel) would qualify under HeyHomie’s clarification in post #11, since there are walkways in them for evacuation in case of an emergency.
The Posey Tube between Oakland and Alameda (California) has a shared bike/pedestrian path next to the traffic lanes, which I have used a few times; there’s something creepy about walking through it, and the traffic exhaust is terrible!. The parallel Webster Tube is auto/bus/truck-only, but has an emergency-use sidewalk.
[I get the feeling that the OP is more after self-contained breathable-air structures, rather than tunnels, but hey, you take what you get. ;)]