Pfft! Forget all of those complex pillar schemes. You can build a 'natural' island from scratch!
Ingredients:
One lava vent
One elevated chute
One ocean
Preparation:
Extend the chute from the magma vent, out over the ocean to the desired location for your new island. Begin magma pour. The magma will harden into a layer of obsidian on contact with the water, and collapse onto the sea floor. The result is a pillar of obsidian leading straight up to the water's surface (since the layers that you pour collapse straight down onto one another), which then mushrooms out into a platform.
Theoretically, I suppose that you could get a more convincing sort of pillar if you cut off the magma flow one level below the water surface, and relocated to an adjacent square. The timing would be difficult (although a bold miner could always adjust your mistakes with a channel, if you could find a way to get him out there), but you could possibly even make a hollow pillar which you could then pump out to access the sea floor without tunneling over from land.
EDIT: Err, actually, for accessing the sea floor through your pillar I guess you'd need a solid core, since you can carve downwards, but not construct downwards.
EDIT #2: Aha! Dash timing! Maybe you could make a mold...
You'd need a lot of bauxite, but perhaps by making a 3X3 row of staircases (up, down, up-down- AFAIK it wouldn't matter, since you'd only be using them because you can place hatches on them, and walk on them), placing floor hatches over the staircases, and placing a floodgate directly on the edge of your mold, like so:
╔═══╗
║эээ╠═══
║эээ█+++
║эээ╠═══
╚═══╝
(Where э= a floor hatch and █ a floodgate because I couldn't quickly locate the appropriate symbols)
Then maybe you could fill your mold, close your gate, and open your hatches to drop a clean 9X9 section of pillar onto the sea floor below. I reckon you'd have to find a flat patch of ocean- but that's not really so hard.