Pool in a submarine A common theme in aquatic science fiction is the submarine pool/access to the ocean. That terrible TV show Seaquest had it, The Deep & Deep Blue Sea (Samuel L Jackson is standing in front of it when the shark chomps him). My question is how this could possibly work? From what little knowledge I have, I'd say the cabin where the pool resides would have to be pressurized   to the water at that depth. The implications are that you'd have to pass through an airlock to get to the room, and that it would only work to a certain depth. 
Is this correct, or it too far to the "fiction" side of the science fiction axes?
 A: 
The implications are that you'd have to pass through an airlock to get to the room, and that it would only work to a certain depth.

Correct x 2.
You have 2 choices with an underwater habitat - build it really strong to take the pressure, or just pressurize it and you can make the whole thing out of plastic.
Submarines chose the former, as they may need to surface quickly and don't want the crew exploding when they open the hatch. Exiting a sub below a certain depth is not possible, and a small hole can create a steel-cutting jet inside. Descending below crush depth is ..... bad.
If you want to put something on the bottom with a convenient door you just raise the internal pressure of the whole thing to match the external pressure. Done for underwater construction all the time. Mixing the gases is important but also not complicated, the crew goes to the surface in a pressurized diving bell and spends a lot of time decompressing. Accidents do happen, and are rather spectacular. Look up "Byford Dolphin" in the usual place for the gory details. In theory this will work all the way down to the Mariana Trench, if you can get a powerful enough air compressor.
A: Is there a particular way that you think this scheme will fail?
Rather than have an airlock with that particular portion of ship, you can simply pressurize the entire vessel.  There are practical reasons why you would not want to do this at great depths (related to how much gas you use and toxicity), but the problems are not related to how the access works.  
Given sufficient gas, it will work to any depth.  You simply need the air in the vessel to be at the same pressure as the water at the point of access is.
This access is called a moon pool.  The wikipedia page has some examples of its use underwater. Moonpool habitat examples
A: You can do it without raising the internal pressure.  First you have your pool, at 1 atm or ambient inside the vessel. Then you have a wet chamber which can be sealed at both ends.  Enter the pool, then only the inner door of the wet lock opens, enter this area, and close the door to the moon pool behind you.  When the internal door is shut you can open the external door.  External water pressure never reaches the internal pool. The mass / volume of water inside the wet lock won't change so it won't push back into the atmosphere of the habitat. You couldn't use this as an unprotected diver since you would explosively decompress every time you re-entered the vessel interior (because it's at 1 atm and you have just entered from a significantly higher pressure zone)  But it would serve to protect the atmosphere inside the vessel and can be used with 1 atm suits or subs.
