The first step in the Deep Sentinel program, one of the most ambitious ocean-engineering projects in decades, will be completed this year. This initial step, called Vanguard, will shelter three aquanaut-scientists at depths of 100 meters. Eventually, the project will create a modular underwater habitat system that will let scientists live underwater for weeks at a time.
Space is hard to get to, no gravity, and there’s radiation.
Underwater has high pressure, corrosion, and no natural lighting.
When you get an air leak in space, you find the hole and patch it. When you get a leak underwater, you don’t have to worry about it at all because it takes care of things in microseconds.
Just like with space you can build in redundancies though. You don’t have to be all Titan about it.