Hmm. The pressure inside a Merlin engine is about 1410 psi (about 88 atmospheres) and the temperatures inside the engine reach 2200 C. The turbopumps in the engine spin at about 3600 rpm and produce about 10,000 horsepower, which is just used to pump fuel into the main combustion chamber. Each Merlin engine produces about 200,000 pounds of thrust and is still reusable.

Just starting a rocket engine is a complicated dance that can easily turn out badly if not handled with extreme precision. Sorry, but I don't think there is any way one can equate the complexity of a diving bell with that of a rocket.

Are you forgetting that little something at the back of rocket that is so hot it can melt itself?

Even managing propellant tanks is complicated dilema. Take a big jug of water that is half full and slosh it around. Its one of thousands small things that needs solving. And solutions need to be implemented with as few grams as possible.

James Cameron spent 10M to built his submarine. It is impossible at present time to build a orbital manned rocket for that amount. Therefore the submarine is easier to design and build.

Relevant Futurama scene: https://youtu.be/7GDthiBGMz8

A spacecraft has to deal with zero to one atmospheres.

A rocket has to deal with rocket thrust/structural cross-section units of pressure. I'm neither a engineer nor informed about the structure of a Falcon 9, but that number is > 1 atm even if the entire cross-section is structural.

Make that 450 actually :)