@ellmer has built a couple of them. It's an impressive accomplishment.
There are some excellent qualities to such a structure. The materials are reasonable cost. Concrete's biggest strength is in compression, and the torpedo like shape is mostly self-buttressing (spheres are the ultimate self-buttressing shape, but cylinders come close and are a more usable interior for practical purposes).
Recent knowledge advances in Geopolymer and Basalt FRTP rebar may make it an even better structure due to better water/salt resistance.
What about construction techniques? I have never seen any good description of how the things were built. What is the difficulty level for
-the third world shipyard
-the industrial world shipyard
What tools, equipment, and facilities are needed?
Submarines are great when they are below the waves, but only nuclear powered submarines can submerge for more than a few hours. On the surface, submarines have a reputation for being miserable wallowing pigs.
People don't thrive in tight enclosed environments. Naval submariners are carefully chosen for exceptional tolerance for the environment, and despite spending months at a time, don't live in submarines and must rotate out at intervals. How does this relate to a seastead that is normally submerged?
Granting the engineering benefits to a torpedo shape, how does that actually relate to a practical seastead?
Granting that concrete cylinders are great in co press ion, how many duty cycles of compression and expansion does concrete have in it before failure.
Does the concrete sub equalize pressure or rely upon structure to resist crush? If equalizing pressure, there are a whole slew of other hysiological issues to overcome.
Submarines don't allow the crew to interact with their environment the way ships do, they just insulate crew from the environment. How does that advance living and making a living at sea? Other than a very few, very limited tour subs, submarines are currently only in major use for austere exploration and austere military purposes. There aren't any recreational live aboard family subs in common production I am aware of.
All of the arguments against ships as seasteads seem vastly magnified when it comes to submerged structures and vessels, unless they are fixed to a shallow bottom and open to the top.