'splain.
1) Dig out big chamber underground. The roof of the chamber will be the sea floor.
2) Build fancy dwarf city in chamber. Make sure all structures have roofs that are a z-level thick, and that the whole complex is airtight.
3) On the surface, build a big ring-shaped gantry suspended above the ocean that exactly lines up with the perimeter of the chamber.
4) Collapse the entire ring simultaneously.
5) The ring hits the seabed, which is only 1 floor thick rather than a z-level thick, so the whole roof of the artificial chamber collapses simultaneously.
6) All the structures in the fancy city have roofs that are a z-level thick, so they survive the sea floor collapsing on top of them.
7) My frame rate collapses to a number so close to zero that mathematicians will debate on whether there's any practical difference between my framerate and zero, as an area several z-levels deep and covering a significant portion of the map is flooded by the ocean.
Catastrophic amounts of fun when it turns out I forgo to install a couple of windows, and the whole project (which naturally I've already moved my dwarves into) is flooded.
Assuming that my computer can survive the framerate problem and my dwarves can survive my incompetent building practices, the end result
should be a bunch of buildings sitting on the bottom of the ocean.