Fairly expensive, as you’d need a new disposable submarine for each execution.
Most execution is actually pretty barbaric. The multi drug cocktail used for executions in the US is a horrible system that was designed by a non-doctor. The first drug knocks you out, the second drug paralyzes you, and the 3rd drug stops your heart. Problem is if they don’t give you enough of the first drug, you can be awake while your heart is being stopped which is apparently quite painful, but since you’re immobilized you can’t communicate that at all.
The only humane way I am aware of to kill a condemned person would be have them breathe pure nitrogen. Nitrogen is all around us- the air we breathe is 80% nitrogen so the body doesn’t react negatively to it. But without that 20% oxygen, the brain asphyxiates and dies in just a few minutes. The prisoner would simply pass out and not wake up.
This protocol is often used to euthanize laboratory animals as it is considered among the most humane.
It’s pretty fucked up that lab rats get a better death than humans.