Oh, I know that stuff- Followed chemistry/physics until I went to college, so I know how pressure works- The thing I'm not sure of is how the higher pressure would have effect on a body- It makes sense to me that it would affect breathing, but I could totally be wrong there. It could also be that currently you guys lungs should have im/exploded,but artistic license...
You're right that the pressure could technically be anything though, because magic- If it were pure physics, either the bubble would burst or indeed probably kill you guys upon entry.
That being said, it's intentionally high- They could probably make it so the pressure would be the same as it is on the surface... But that'd be one big change when they step out of the air bubble, wouldn't it?