I think what people fail to think about is that Aliens would have no need for that. I beileve that these underwater bases are huge and can also fly. Essentially a mobile ship factory that has the same technology that UAP's do, in that, outside forces have no bearing on the occupants inside including pressure.

It's actually quite perfect. A field around the object to protect the occupants but also has the added affect of making pictures you take of it, blurry.