If there are people living deep underwater swishing around in nice submarines, how will they provide value to normie landlubbers?
http://theoildrum.com/node/4558 all but rules out the possibility they would harvest minerals. This more recent article possibly rules lithium back in: https://www.mining.com/scientists-develop-cheap-and-easy-method-to-extract-lithium-from-seawater/
That's extracting minerals from the seawater, but what about from the sea bed?
Then there's fish, of course. I wonder if there's some deepsea fish that's really yummy or valuable.
Possibly they don't provide a good, but a service or manufacturing process that requires high pressure.
There's always research and basic science, but what specific fields? Maybe some sort of bioprospecting?