Need some help building an underwater lair.

[Mrpops]

Is there any spell that acts as an airlock, or stops water in some way?

[CriticalQuit]

I don't know if any spell specifically does that, but you can always make something up. When it comes to environmental magic, it's more fun to come up with something interesting than to confine yourself to only what's in the books, as it's mostly for player use.

That said, when you do come up with an effect, make sure you come up with the proper parameters (like what comes up if someone uses Detect Magic on it).

[Fake Healer]

A reverse permanent Aqueous Orb? Otherwise you could just have the cavern/tunnel that they enter through arch up to the lair, providing a natural formation that allows for air to remain in the chamber....tube leading into a diving bell. The bell part wouldn't fill but the tube(tunnel) still provides access. Replenishing the air supply would be more of the issue unless you make up some lichens and plants that grow in the area and help replenish the supply, or use magic, or pump in fresh air somehow.

[Tangaroa]

Control Water could make an airlock in a cave.

[Ciaran Barnes]

You're really outside the realm of "what can the rules do for me". This is GM fiat area. Ask what would be neccessary, and do your best to ssatisfy the requirements, no matter how arbitrary.

For the sake of fluff, not an actual mechanic benefit, invent some kind of machine or magic item that creates/brings in fresh, breathable air that not only keeps creatures from suffocating, but also keeps water from coming too far into the lair.

[Lincoln Hills]

Refer to the airy water spell (commonly seen in 2nd edition; the 3rd edition version appears in Stormwrack.) But when I created a giant Bioshock-style submarine city for my campaign, my players weren't surprised to see a magical barrier at the entrance that blocked sea water while admitting creatures. (They did feel bad for all the fish that had discovered it before they did.)

Alternately, if you don't mind canal-style airlocks, you can combine three wall of force effects; step into the central area and the two outer walls go up; a mechanical or magical pump fills or empties the central zone; then the middle wall goes down long enough to step across it; then it goes up; lastly, the outer walls vanish. (It would be clearer in cartoon form.)

One addendum based on my own experience in designing my city: at any serious depth, sea water provides a tremendous amount of hydraulic pressure that can easily be exploited in making murderous traps. Also, sliding doors are better-suited to your lair than doors that open on hinges. (In the event of leaks, sliding doors still work.)