A guest at Jules Undersea Lodge scopes out the ‘hood.

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.

Jules Undersea Lodge Built 1972 Aquarius Undersea Lab Built 1986
SPECS Sits at depth of 30 feet, measures 50 by 20 by 10 feet, weighs 300,000 pounds Sits at depth of 62 feet, measures 43 by 9 feet (cylinder), 164,000 pounds
ENTRY 100-foot underwater swim to the lobby 9-mile boat ride, then a dive to front door
AMENITIES In-room dining, ISDN Internet service, 42-inch-diameter viewports Microwave meals, wireless LAN for Web access, 26-inch-diameter viewports
THE GOOD Near shore lagoon, average visibility of 30 feet Spectacular coral reef, average visibility of 60 feet
THE BAD No smoking, booze, or sunsets Pressurized toilet, 17 hours of decompression at end of 10-day mission