While NASA was working to put a man on the moon, we also started thinking more about the possibility of colonizing on our own planet. It might even be a good test-drive to find out how well we could live on the moon, plus we could test out living in an isolated environment and conduct research experiments.
The U.S. government has been involved in several underwater habitats. First the U.S. Navy built SEALAB I, an experimental underwater habitat, in 1964 and sunk it 192 feet (58 meters) below sea level. SEALAB II and III followed. Tektite, built by General Electric and funded by NASA, the U.S. Navy and the U.S. Department of the Interior, was another research facility in the late 1960s.
There were totally private ventures, too. Famous marine explorer Jacques Cousteau built the Conshelf habitats in the mid-1960s (the deepest of which was about 336 feet or 102 meters). It didn't take long for innovators to think about longer-term living, though. At the 1964 World's Fair in New York, the Futurama II exhibit presented by General Motors included an undersea hotel. In 1971, a group of British investors created models promoting an entire city underwater, which would have been built in the Black Sea and called Pilkington Sea City.
There are still underwater research facilities, and a few underwater hotels, but no cities. Why not? One big issue is decompression sickness, or "the bends," a potentially fatal condition related to the fact that water exerts twice as much pressure on our bodies as air. Maintaining the right atmosphere, as well as the logistics of providing supplies, is complicated and expensive. That's probably why a night in the Jules' Undersea Lodge in Key Largo, Florida (a remodeled underwater research habitat built in the early 1970s) costs upwards of $500 and it's only about 30 feet below.