+1 on Fifi's remarks...
Much like a pressurized airliner, the metal hull/fuselage structure absorbs the pressure difference between outside and inside environments and the interior pressure remains at a comfortable level... the difference between the submarine and airliner is which side of the skin is being pressurized, and the metal used for the structure.
Because a circular cross section distributes the applied pressure evenly, it is the first choice for airborne or underwater pressurized vessels, and since a sphere is a 360 degree circular cross section, it is why many deep sea submersibles' pressurized compartments are that shape.
A bit of off topic trivia -
At airliner cruising altitudes a 3 x 6 foot passenger entry door exerts more than TEN TONS of outward force against its frame. (3*6*144*8.5)... Similarly, the 18 x 18 inch window has about a TON AND A HALF of air pressure force working to push it into the stratosphere, taking the nearby occupant(s) with it
... "We advise you keep your seat belt fastened while in your seat. Now, just sit back, relax and enjoy your flight"
