I’d think with a larger structure you may be able to use foamed-concrete instead of polystyrene to fill air-cavities and create positive-buoyancy and thus unsinkability. Thing is, the more weight being supported, and the deeper the hull, the greater the pressure water would shoot in if there were damage. Which is probably why styrene wouldn’t work for depths–it might just cut right through it and compress it. But closed-cell concrete foam might continue to have the kind of compressive-strength concrete is famous for, resisting water-pressure. But because of its weight would make for large effective displacement chambers.
But I doubt you’d have much success with dish-soap. I’m not sure what foaming agent is actually used to make foamed concrete. As a guess, what you want to make a porous concrete would probably be to make the concrete far more sticky by adding more lime to it, not detergent! Then, take some compressed air and fluff the mixture, blowing air into it somehow.
In any case, I think just having air-pockets wouldn’t be enough. You could try mixing in some water-proof but light additives. Like maybe adding styrofoam particles to the mix, say 30% or so.
The problem with pillars is that they tend toward rocking and rolling motions and because of this are rather uncomfortable.
You see them in oil rig designs because they are working vessels, not comfort vessels, and the guys that live on them more put up with them than enjoy their time there.
For maximal comfort you want to laterally spread the floats, centralize the weight, creating almost an inverted pyramid shape or a T shape rather than an I shape.