Since 3-d space could be be much more efficiently utilized underwater, there might be some sort of layered appearance to interior structures, and if the aliens are travelling horzontally much the time, it might give the visual impression that the city appeared somewhat porous as there could be multiple openings to different areas of dwellings and sections of architecture.
This might have cities "growing" vertically at the same rate as they grow horizontally.
Just a thought.
-STS
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