>>1727234Structural failure is probably your #1 concern with a 1-atm sub, because even if you were to get stuck on the bottom and had to bail out, you could open a flood valve in the bottom of the sub to let water in and equalize pressure to open the hatch. Same thing if you have a leak.
Because you're under external pressure, the failure mode is buckling, which happens without warning and is catastrophic. You probably would want to add internal ribs to stiffen the tank, and not go below like ~200 ft
