There are more than 3 million certified scuba divers in the world, yet only two places to overnight beneath the sea. Both habitats are off the coast of Key Largo, Florida, and both were born as deep-sea labs.

But that's where the similarities end. One of the habitats, the Jules Undersea Lodge, is now a fully operational hotel (with rooms going for between $250 and $350 a night), while the other -- a lab called Aquarius owned by the National Oceanic and Atmospheric Administration (NOAA) and operated by the University of Carolina at Wilmington -- could change how we train astronauts for trips to the space station.

In October, NASA and NOAA conducted a weeklong experiment aboard Aquarius to explore using it to simulate space missions. Both agencies expect to benefit from the experience, since the extreme environments of sea and space present similar challenges for aquanauts and astronauts.

I'm one of a few people to have bunked down at both places. Here's how they measure up under the (intense) pressure.