It's not just that. Exposed electronics will work fine in a vacuum, not so much under water. So lets say the cabin is pressurized and the engines are completely airtight *and* watertight (yes, airtight and watertight are different and the seals for freshwater and saltwater are different too), what about all the avionics, hatches? hydraulics, etc? It's pretty ridiculous that a plane/spaceship would also be designed as a submersible as well.. yes, even a plane/spaceship that transforms into a robot.