...
As an experienced obsidian/oceanic engineer I think it's easier to cast a solid block and mine out all the inside. Building gantries for the ring to hold the shell takes a long time due to linear construction, I think it would be much faster to build lots of floor.
Also pumping out the water takes ages, and you need more gantries for the pumps. Unless you sacrifice a dwarf and drain out the bottom edge of the map or something.
Beware: Building destroyers can destroy windows. Build with green glass blocks.
Also remember some sea creatures have building destroyer.
Time to drain interior: short (see below)
Time to mine out solid block: long, proportional to area of city, limited by number of miners.
Construction speed of anything: limited by surface area for masons to stand and the number of masons that you have, plus the amount there is to build. Building a ring requires fewer blocks than building a large square area.
You need to drop the ring at the end anyways in order to sever the connection to the sea floor.
I never said that I pumped out the water. I used my magma supply to ignite an iron bin full of coal bars, and dropped that in the interior to boil away all the water. Even if I wanted to mine out on the sea floor, that doesn't require a dwarf sacrifice - you can have your miners survive if you do it properly.
Having glass windows in the fort is more fun. It's more likely that a dwarf will break/topple one in a tantrum spiral. It also justifies having backup secondary tunnels between buildings (instead of the standard skyway tubes).