Nuclear submarines (fission) are able to not only cruise for months underwater, they are able to make their own air and own water. They can withstand immense pressure differentials between habitable interior and hostile exterior.
What other vessel is closer to the design requirements for space? Add food producing (e.g., hydroponics) and waste recycling (already a hot topic) and you’ve got a spaceship. You’ll still need a better engine to get places (e.g., fusion)… but still!
Image courtesy Axeman3d