What happens when you inflate a balloon on a submarine?

Would it be harder to do? Are submarines pressurized like that? It always seems that way in the movies (Like, they have to pressurize the submarine to a higher PSI than the water-pressure around the ship)?

Am I just misunderstanding how pressure and submarines work? I have this image now of going to deep, and a rivet shoots out and pops a super-dense balloon someone blew up at under 2000 feet.