I am developing an ocean world for a story involving marine mammals, and the biggest issue I cannot settle on is whether to have deep or shallow civilisations.
The main concern is that their architecture will include heavy use (perhaps even exclusive use before they develop air and water tight seals) of moonpools and airbubbles, for the purposes of this question assume that this is a non-negotiable factor when formulating an answer.
How would airbubbles/moonpools (where the top portion of a room is air) be achieved deep in the water, are they even possible? I am asking in regards to scientific possibility and practicality, not how would the species literally find the air and transport it, for which I already have some ideas. Assume the species has access to plants that recycle the air aswell.
The planet has earth-like astronomical and meteorological conditions but is not earth and not close to fully formed in my mind yet, so feel free to adjust the planet's atmospheric composition or the composition of the seawater whilst keeping it fundamentally earth-like if that provides avenues to a solution.
EDIT: I imagine that the species would initially build primitive open-entrance shelters in shallows where quick access to the surface is possible. As their ability to farm coral (including selective cultivation of watertight species for instance) and manipulate it as a building resource increases I envisage that they would have less reliance on the shallows and move deeper to exploit resources in the depths, just as humans did in regards to coastal civilisation spreading inland.
I am fairly set on their buildings being for the most part coral domes entered from the base, like igloos, and featuring moonpools and ledges to provide living space and a dry area for whatever purposes that may serve. The entrance into the dome would be swam through and the surface of the moonpool above the entrance.