-I don't want to have to go into the physics of this, but this sort of thing would work just fine only ~100ft below water.  Yea the air pressure inside the building would technically be higher due to the water pressure from the "Sub-Entrance" pushing on all of the air inside, but not to a level that would be detrimental.  The pressures at those depths are certianly within the engineering capabilities of human construction.  Large ships around the world have these sorts of setups in their hulls all of the time. Jibberish81