What has kept human kind from putting hotels on the ocean floor in all parts of the world?

Money would be the correct guess. The huge amount of pressure at even a modest depth like 100 feet is a large engineering challenge. You need a very strong structure to withstand such pressure, and that means top-of-the-line materials and build quality. Hence it costs a lot.

The average ocean depth is 3700 meters. The largest hollow structure that has been designed to reach this depth (and a little beyond) is Alvin's titanium pressure hull. It's a 2 meter inside diameter sphere which can barely contain 3 people and 72 hrs life support. Once you get there, you have a view of 100 meters or less from a viewport smaller than your head. Nothing even as large as a small military sub has ever been built to reach seafloor depths. Space is a much less hostile environment, with infinite views and zero-gee as attractants.

Added later -Luxury subs, which access the shallow (less than 1000 ft depth) ocean where there is some light to see by, do exist. They are VERY expensive and, despite the article title, don't get to seafloor depth - just the continental shelves.

I'm from Planet 12 and I find you people ........ ummmm ... interesting. And I've found that after many years of trying to cope with you people I have learned to adapt. It's not easy since I have to "dumb it down" a LOT to even talk to most of you. Once 2012 hits and you reach enlightenment and you ascend to the 4th Dimensional beings you are destined to be it will be easier to tolerate your absurb stupidity over the most basic things such as what YOU call Quantum Physics and the like. But really ... you call yourselves intelligent beings and you elect a President of the USA like ... what's that idiots name? Oh yeah ... Tree ... or Branch .... or Bush ... yeah ... that's it ... The Bush. Come on now. You need to get rid of that right away. And what's with all this Global Warming and wars and constant vengeance and hateful remarks from your "Christians". Aren't they supposed to be Christ-like? So what's with all their bickering and trying to run other people's lives and theri constant judgments even though their Bible tells them NOT to do that and yet they refuse to obey their own God and Lord and his Sacred Books? This is NOT intelligence. No biggy ... I only have a few more years and then I go back home. Once I make sure that your 2012 ascension goes off without a hitch everything will be ok but I worry about intervention from the Drakonians. You know them as "Lizard People" .. you know ... those Bushes, the Windsors, and most of your world leaders. Did you know that your planet is being run by a bunch of space aliens? Yeah ... they took over during the Sumerian Empire and they have been controlling you thru the banks, govt's and religions for about 5,000 years. GEt a clue Earthicans ... toss off the shield of ignorance and fight for your independence. 12/20/2012 will be the new World Independence Day so get ready and open your minds. :-) Peace EArthicans

Lack of demand. Not enough people want to hang out under the sea to justify the cost of building such a thing.

They do exist:

http://www.jul.com/

http://www.uhotw.com/HotelDetails.aspx?ClassID=30&...

http://www.poseidonresorts.com/poseidon_flash.html

http://www.designbuild-network.com/projects/Hydrop...

http://www.popsci.com/popsci/technology/ddd2b25900...

Reminds me of the Green Onion. Glub Glub.

http://www.imdb.com/title/tt0064417/

the space is empty whereas the sea has alot of mass.

more the mass = lot of factors to deal with while building something

It's too wet for humans, and the sharks and whales always skip out without paying their bills.

I am gonna have to go with extreme pressure. The deeper you guy the more psi (pressure per square inch).

Try asking a cab driver to take you to one

Atmospheric pressure makes it very hard to keep it secure