They're certainly not airtight. There's intentional leaks in a pressure cabin. This is used to regulate the pressure, there is constant engine bleed air flowing

They're certainly not airtight. There's intentional leaks in a pressure cabin. This is used to regulate the pressure, there is constant engine bleed air flowing in. If you allow more air than it leaks out the pressure will rise, if you lower the airflow it will drop. This mechanism also takes care of refreshing the cabin air. There's also some catalytic conversion going on to remove the high ozone content that's present at high altitudes. But I think it's pretty hard to ditch a plane without damaging the fuselage to some extent.