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Remembering Sealab
An anonymous reader writes "'Some people remember Sealab as being a classified program, but it was trying not to be,' says Ben Hellwarth, author of the new book Sealab: America's Forgotten Quest to Live and Work on the Ocean Floor, which aims to 'bring some long overdue attention to the marine version of the space program.' In the 1960s, the media largely ignored the efforts of America's aquanauts, who revolutionized deep-sea diving and paved the way for the underwater construction work being done today on offshore oil platforms. It didn't help that the public didn't understand the challenges of saturation diving; in a comical exchange a telephone operator initially refuses to connect a call between President Johnson and Aquanaut Scott Carpenter, (who sounded like a cartoon character, thanks to the helium atmosphere in his pressurized living quarters). But in spite of being remembered as a failure, the final incarnation of Sealab did provide cover for a very successful Cold War spy program."
One REALLY big difference between undersea and space is air pressure. If you want people to be living near 1atm of pressure then in space you have to deal with at most 1atm of pressure on your hull. Underwater you're dealing with more than 1atm of pressure before you reach depths on par with a big swimming pool. That means you need a lot less structural strength in your spacecraft.
All the messing around with gas mixtures undersea is about trying to work at higher pressures to cut down on that disadvantage, but it gets really messy - people are designed to live at 1atm on 20% O2. In space that is fairly easy to provide, and deep underwater it is almost impossible.
Now, in space you have lots of other issues to deal with I'll grant you, and the cost of moving around is pretty high too (well, maneuver in space is cheaper per mile than underwater, except that stuff is thousands of miles apart so you do a LOT more of it and once you're close to a gravity well you build up kinetic/potential energy and changing your energy state is much harder). Underwater you can just use buoyancy to do half the work.